Before diving in, we first need to establish what a prospect is, and since there are no hard and fast rules, I can sort of make up my own.

Industry standard is anyone who falls short of the rookie eligibility thresholds, meaning 130 at-bats or 50 innings, and that works well enough for preseason lists. But this list needs to live for a while. I can't have Jake Bauers disqualifying himself a week after I put him on.

Plus, it's just boring. "Hey guys, you want to hear more about Bauers or Willy Adames or Kyle Tucker?" As if they haven't been dominating the Fantasy Baseball headlines already.

So my rule for this midseason piece is even more straightforward: If they've appeared in the majors, they don't qualify. Unfortunately, this eliminates some high-profile types who appeared only last year, such as Victor Robles and Francisco Mejia, but in case you needed me to tell you, they'd both be in the top 10.

Something else to keep in mind: There are no 2018 draft picks here. Until we get professional data and scouting reports for them, there's just too much mystery there. The chances of any of them cracking the top 25 straight out of amateur play are pretty slim anyway.

And one last rule! This list is tailored for Fantasy Baseball purposes, which means some extra credit was given to proximity to the majors and almost none to defense (except to the extent that it might limit a player's full-time status down the road).

Got it? Now, let's make some magic happen.

All stats are updated through Tuesday, July 10.

1. Vladimir Guerrero, 3B, Blue Jays

Age: 19
2018 levels: Double-A
2018 stats: .407 BA (204 AB), 11 HR, 1.124 OPS, 20 BB, 21 K
It's ridiculous to think Guerrero could be better than his Hall of Fame father, who was regarded as Fantasy royalty not so long ago. But good grief, he's been Ted Williams at Double-A — and at an age when he shouldn't even be playing at Double-A. Having missed the past five weeks with a knee injury, he's not exactly on the verge of a promotion, but you can't be surprised if he eventually forces the issue.
Second-half callup is ... possible.

2. Eloy Jimenez, OF, White Sox

Age: 21
2018 levels: Double-A, Triple-A
2018 stats: .313 BA (246 AB), 12 HR, .912 OPS, 23 BB, 44 K
In a world where Guerrero doesn't exist, Jimenez is the primary source of our salivation as we ooh and aah over his light-smashing blasts. Though slowed by a groin injury recently, he's on the fast track to the majors now, making every stop look too easy with his unique (but not Guerrero-like) blend of contact hitting and power.
Second-half callup is ... likely.

3. Fernando Tatis, SS, Padres

Age: 19
2018 levels: Double-A
2018 stats: .291 BA (330 AB), 15 HR, 14 SB, .880 OPS, 33 BB, 105 K 
Another second-generation talent, Tatis lacks the polish of the two names ahead of him but has dominated beyond what the full-season numbers suggest, having recovered from a slow start to hit .338 with a 1.008 OPS in his past 58 games. Provided he sticks at shortstop, he has the potential to be another Manny Machado type.
Second-half callup is ... unlikely.

4. Forrest Whitley, SP, Astros

Age: 20
2018 levels: Double-A
2018 stats: 0-1, 4.29 ERA, 1.05 WHIP, 21 IP, 8 BB, 30 K
Whitley's season was sidetracked first by a 50-game suspension for (non performance-enhancing) drugs and, more recently, a strained oblique, but there's little doubt he's the top pitching prospect in baseball, dominating hitters with two wipeout breaking balls and five pitches total. He could be the most exciting pitcher call-up since Noah Syndergaard, and the suspension has left him with plenty of innings for the stretch run.
Second-half callup is ... possible.

5. Nick Senzel, 3B, Reds

Age: 23
2018 levels: Triple-A
2018 stats: .310 BA (171 AB), 6 HR, 8 SB, .887 OPS, 19 BB, 39 K
Just when he had moved past his latest bout with vertigo and gotten back to swinging in the same way that put him on the short list of prospects eyeing a quick promotion this spring, Senzel went and tore a tendon in his right index finger, ending his season. He may return next year as the Reds' starting second baseman, depending what happens to Scooter Gennett, and while there's some question as to how much power he'll provide, his hit tool is off the charts.
Second-half callup is ... not happening.

6. Brendan Rodgers, SS, Rockies

Age: 21
2018 levels: Double-A
2018 stats: .273 BA, (311 AB), 17 HR, 11 SB, .854 OPS, 24 BB, 71 K
Maybe Trevor Story prevents Rodgers from playing the position he was drafted to play, but the former third overall pick has been seeing time at second and third base just to keep the Rockies' options open once he's ready. The prospect of playing at Coors Field should more than make up for his shaky plate discipline. 
Second-half callup is ... possible.

7. Jo Adell, OF, Angels

Age: 19
2018 levels: low Class A, high Class A
2018 stats: .317 BA (281 AB), 17 HR, 11 SB, .966 OPS, 20 BB, 76 K
Though considered one of the most promising first-round selections in the 2017 draft, Adell's power tool has come through sooner than anyone could have hoped. A supreme athlete and genuine five-category threat, Adell is on the path to becoming a superstar and may find himself in a place much like Guerrero a year from now.
Second-half callup is ... not happening.

8. Bo Bichette, SS, Blue Jays

Age: 20
2018 levels: Double-A
2018 stats: .267 BA (344 AB), 8 HR, 26 SB, .757 OPS, 30 BB, 63 K
Baseball is poised to win back viewers on '90s nostalgia alone, because Bichette is yet another second-generation talent who's about to take the league by storm. His numbers aren't as wild as they were a year ago, but he's young for his level, has added more of a speed element to his game and continues to unleash a fierce Josh Donaldson-like swing.
Second-half callup is ... unlikely.

9. Brent Honeywell, SP, Rays

Age: 23
2018 levels: has not played — injured
2017 stats: 13-9, 3.49 ERA, 1.24 WHIP, 136 2/3 IP, 35 BB, 172 K 
You'll see some rank lists that dock Honeywell more than this one for his year lost to Tommy John surgery, but given that procedure's favorable track record and the fact he has already mastered the minors, I mostly interpret it as a frustrating delay. Kid has off-the-charts polish and a legitimate five-pitch arsenal that includes a screwball.
Second-half callup is ... not happening.

10. Michael Kopech, SP, White Sox

Age: 22
2018 levels: Triple-A
2018 stats: 3-7, 4.70 ERA, 1.43 WHIP, 82 1/3 IP, 56 BB, 111 K
In terms of pure stuff, Kopech might rate ahead of even Whitley, having been clocked at 105 mph once. But after seemingly getting his control in check down the stretch last year, he has fully regressed this year, to the point you have to be a little concerned about his long-term standing. Then again, Blake Snell eventually figured it out.
Second-half callup is ... possible.

11. Royce Lewis, SS, Twins

Age: 19
2018 levels: low Class A
2018 stats: .318 BA (283 AB), 8 HR, 19 SB, .855 OPS, 24 BB, 48 K 
The first overall pick in last year's draft has been as advertised, showing few weaknesses in his game at the lower levels. He should have no trouble sticking at shortstop and profiles as the sort of player who'll always be asked to steal bases, giving him a ceiling not too unlike Trea Turner.
Second-half callup is ... not happening.

12. Luis Robert, OF, White Sox

Age: 20
2018 levels: low Class A, high Class A
2018 stats: .293 BA (75 AB), 0 HR, 5 SB, .745 OPS, 8 BB, 22 K
Robert dominated Cuba as a teenager but hasn't gotten much of a chance so far in the minors, mostly because of a thumb injury that keeps flaring up. His skill set has drawn comparisons to Eloy Jimenez, though he's faster and maybe less contact-oriented, and nobody's doubting the upside despite his limited exposure so far.
Second-half callup is ... not happening.

13. Jesus Luzardo, SP, Athletics

Age: 20
2018 levels: high Class A, Double-A
2018 stats: 8-4, 2.30 ERA, 0.96 WHIP, 78 1/3 IP, 20 BB, 99 K 
A first-round talent who fell to the third round in 2016 because of Tommy John surgery, Luzardo has made quick work of the minors so far, continuing to thrive after a mid-April promotion to Double-A. His past six starts: 31 innings, 15 hits, four walks, 36 strikeouts ... and one earned run.
Second-half callup is ... unlikely.

14. Mitch Keller, SP, Pirates

Age: 22
2018 levels: high Class A, Double-A, Triple-A
2018 stats: 9-3, 3.65 ERA, 1.28 WHIP, 98 2/3 IP, 39 BB, 91 K
Keller's numbers don't jump out the way you'd expect for a prospect of his caliber, but with easy velocity and plus command, he certainly has a path to greatness. He's still working to develop a changeup, which may explain some of the discrepancy, and is possibly still a year away.
Second-half callup is ... possible.

15. Sixto Sanchez, SP, Phillies

Age: 19
2018 levels: high Class A
2018 stats: 4-3, 2.51 ERA, 1.07 WHIP, 46 2/3 IP, 11 BB, 45 K
You talk about easily velocity, Sanchez is the poster child, routinely touching triple digits with smooth-as-silk mechanics. He's still figuring out how to make the most of his secondary arsenal, his strikeouts lagging behind the stuff, but comparisons have ranged from Pedro Martinez to Jose Fernandez to Luis Severino. 
Second-half callup is ... not happening.

16. MacKenzie Gore, SP, Padres

Age: 19
2018 levels: low Class A
2018 stats: 1-3, 4.06 ERA, 1.26 WHIP, 31 IP, 9 BB, 35 K
The third overall pick in the 2017 draft may be the most talented, even if he has gotten some of the least exposure so far. He throws in the mid-90s with plus control, four quality offerings and adds a little deception with a high leg kick. The Padres will take their time with Gore, but he's as projectable as teenage pitchers come.
Second-half callup is ... not happening.

17. Keston Hiura, 2B, Brewers

Age: 21
2018 levels: high Class A, Double-A
2018 stats: .306 BA (320 AB), 9 HR, 27 2B, .858 OPS, 19 BB, 68 K
A balky elbow and the threat of Tommy John forced the ninth pick in last year's draft to spend most of his time at DH in the early going, but Hiura has since transitioned back to the keystone, where his bat profiles as something special. He was an exceptionally productive college hitter who should hit for average and power, making him something like what the Brewers gave up in Scooter Gennett.
Second-half callup is ... possible.

18. Luis Urias, 2B, Padres

Age: 21
2018 levels: Triple-A
2018 stats: .278 BA (316 AB), 6 HR, .802 OPS, 54 BB, 78 K
A fascinating prospect in that the numbers don't measure up to the enthusiasm within the scouting community (apart from his consistently exemplary strikeout-to-walk ratio), Urias will catch Fantasy owners by surprise in the same sort of way Ozzie Albies has. His exit velocities suggest some power is in store, and he certainly makes enough contact to hit for average.
Second-half callup is ... likely.

19. Austin Riley, 3B, Braves

Age: 21
2018 levels: Rookie, Double-A, Triple-A
2018 stats: .306 BA (216 AB), 11 HR, .926 OPS, 21 BB, 72 K
If he hadn't sprained his knee in early June, Riley might have already gotten a shot at the Braves third baseman. But now that Johan Camargo has seemingly settled into that role, the young slugger will just have to continue mashing at Triple-A. There are some plate discipline issues, but since shortening his swing with a move up to Double-A last year, he has been a special hitter.
Second-half callup is ... possible.

20. A.J. Puk, SP, Athletics

Age: 23
2018 levels: has not played — injured
2017 stats: 6-10, 4.03 ERA, 1.25 WHIP, 125 IP, 48 BB, 184 K
You may remember Puk was pitching himself into a big-league job before succumbing to Tommy John surgery this spring. The sixth overall pick in 2016 has some control issues to iron out but has terrific stuff, actually leading all minor-leaguers with 13.2 strikeouts per nine innings in 2017.
Second-half callup is ... not happening.

21. Brendan McKay, 1B/SP, Rays

Age: 22
2018 levels: low Class A, high Class A
2018 stats (hitter): .224 BA (116 AB), 2 HR, .733 OPS, 36 BB, 28 K
2018 stats (pitcher): 4-2, 2.81 ERA, 0.75 WHIP, 48 IP, 6 BB, 69 K  
The minor-league version of Shohei Ohtani has clearly fared better as a pitcher so far, putting together an incredible strikeout-to-walk rate, but his stirikeout-to-walk rate as a hitter still offers room for optimism there. No need for him to commit yet — and he may never have to — but the fact he'll get to so late in his minor-league career assures him a higher chance of success at one or the other. 
Second-half callup is ... not happening.

22. Triston McKenzie, SP, Indians

Age: 20
2018 levels: Double-A
2018 stats: 3-2, 3.05 ERA, 0.89 WHIP, 38 1/3 IP, 9 BB, 32 K  
McKenzie only recently came back from a forearm injury — and of course it's a relief he didn't need surgery — but the strikeout rate hasn't been up to its usual standard so far. He's still filling out physically, so there's a chance he adds some velocity before reaching the majors — an exciting thought given what he has already accomplished.
Second-half callup is ... unlikely.

23. Hunter Greene, SP, Reds

Age: 18
2018 levels: low Class A 
2018 stats: 3-7, 4.69 ERA, 1.34 WHIP, 63 1/3 IP, 20 BB, 80 K 
Greene, the second overall pick in the 2017 draft, may have usurped Kopech's claim as the hardest-throwing pitcher in the minors, occasionally hitting 102 mph. It wasn't totally clear he'd be a pitcher when he was drafted, so the secondary stuff is still catching up. He has a 2.72 ERA over his past nine starts, though.
Second-half callup is ... not happening.

24. Taylor Trammell, OF, Reds

Age: 20
2018 levels: high Class A
2018 stats: .301 BA (272 AB), 6 HR, 16 SB, .828 OPS, 40 BB, 63 K
As much of a football prospect as baseball in high school, Trammell was kind of underdeveloped when the Reds drafted him in 2016, but has he ever made up for lost reps. The plate discipline has been the most impressive part, allowing him to hit for average right away, and he has the physique and raw ability to become a 25-25 type, if not better.
Second-half callup is ... not happening.

25. Yordan Alvarez, OF, Astros

Age: 21
2018 levels: Double-A, Triple-A
2018 stats: .311 BA (180 AB), 12 HR, 14 2B, .962 OPS, 19 BB, 48 K
Alvarez has found another gear as a power hitter this year, which is exciting given his penchant for line drives and unusual strike-zone judgment. It's no wonder the Dodgers and Astros, two of the best organizations at evaluating talent, fought over him coming out of Cuba, with the latter eventually acquiring him from the former in a trade.
Second-half callup is ... unlikely.

Also considered: Kolby Allard, SP, ATL; Peter Alonso, 1B, NYM; Ian Anderson, SP, ATL; Michel Baez, SP, SD; Cavan Biggio, 2B, TOR; Dylan Cease, SP, CHW; Dane Dunning, SP, CHW; Jon Duplantier, SP, ARI; Alex Faedo, SP, DET; Stephen Gonsalves, SP, MIN; Nick Gordon, SS, MIN; Alec Hansen, SP, CHW; Danny Jansen, C, TOR; Carter Kieboom, SS, WAS; Alex Kirilloff, OF, MIN; Brandon Lowe, 2B, TB; Nathaniel Lowe, 1B, TB; Matt Manning, SP, DET; Adrian Morejon, SP, SD; Chris Paddack, SP, SD; Roberto Ramos, 1B, COL; Justus Sheffield, SP, NYY; Christin Stewart, OF, DET; Taylor Ward, C, LAA; Kyle Wright, SP, ATL