The team with this year's top scoring defense ranked 17th in the NFL in points allowed in 2016. They ranked even worse in defensive efficiency, finishing 27th in Football Outsiders' defensive DVOA – 21st against the pass and 30th against the run. (DVOA is Defense-adjusted Value Over Average, which adjusts performance for down, distance, and opponent.) They saw their top cornerback leave for a division rival, then traded their No. 2 cornerback for a slot receiver shortly before the season started (while executing a simultaneous trade sending their No. 1 receiver away in order to acquire their No. 2 corner's replacement), and followed it up by trading a linebacker they'd taken in the second round just last year as well.
But after an offseason coaching change from Rex Ryan/Anthony Lynn to former Panthers defensive coordinator Sean McDermott, the drafting of former LSU cornerback Tre'Davious White in the first round, and the acquisition of cornerback E.J. Gaines to replace the departed Ronald Darby, the Buffalo Bills have allowed just 54 points in the first four games of 2017.
That 13.5 points per game average is a full point-and-a-half better than the next closest team, and it ranks inside the top-40 of all team seasons since the league expanded to 32 teams back in 2002. Not only that, but Buffalo was ranked No. 2 in defensive DVOA after Week 3, and given that the Bills were behind only the Ravens (who just gave up 381 yards and 26 points to the Steelers), there's a strong chance they could take over the No. 1 spot there this week as well.
Making the strong defensive start even more impressive is that the average drive for a Buffalo opponent has started on the 29.3-yard line; that may not seem all that great but it's actually the ninth-worst average defensive starting field position in the league. The Bills have made up for that field position by forcing turnovers. They have seven takeaways on 44 drives by their opponents, good for a 15.9 percent turnover rate that ranks fourth in the NFL. They've also clamped down defensively in the red zone, allowing only four touchdowns on opposing offenses' 12 trips inside their 20-yard line. That's the second-lowest red zone touchdown rate in the league.
The Bills have blitzed the opposing quarterback on only 26 percent of drop backs, a figure well below the league average rate of 30.1 percent, per Pro Football Focus. Despite that relative lack of blitzing, however, the Bills have managed to get pressure at a nice clip, recording a sack, hit, or hurry on 35.9 percent of their opponents' drop backs. That's almost 2 percent better than the league average of 34.1 percent so far this year.
That pressure, generated mostly by Jerry Hughes, Lorenzo Alexander, Eddie Yarbrough, and Kyle Williams, has allowed Buffalo's defensive backs to remain in coverage for less time, and has aided them in limiting opposing passers to just 6.0 yards per pass attempt and a brutal 64.1 passer rating. That's like turning every quarterback into Joey Harrington.
Gaines has allowed only 95 receiving yards on 140 coverage snaps, per PFF, and ranks eighth among 113 qualifying cornerbacks with a 41.0 passer rating allowed. White is not too far behind him with 162 yards on 169 snaps, and a 53.7 opponent's passer rating that ranks 14th among the same group of corners. Even third corner Leonard Johnson isn't too far down the board, checking in 47th with an 85.0 passer rating allowed. That has to be much better than what the Bills expected to get out of their cornerbacks after seeing Stephon Gilmore leave for the Patriots and trading Ronald Darby to Philadelphia right before the start of the season.
Safeties Micah Hyde and Jordan Poyer have been equally impressive in coverage, shutting down over the top routes like posts, corners, and fades; opposing QBs are 7 of 21 with two picks on those routes against the Bills so far.
Buffalo's run defense has not been quite as impressive as its pass defense, but it has still been good. The Bills have allowed only 3.9 yards per carry so far this year, a top-10 mark. They were 9th in rush defense DVOA in three weeks but that was before allowing Devonta Freeman and Tevin Coleman to gain 137 yards on 27 carries, so they'll likely be moving down a bit there.
But the operative question here isn't so much how good the Bills have been so far, but whether they can keep it up. On that front, there are some concerning signs.
| OPP | RUSH YDS | PASS YDS | TOTAL YDS | 1ST DOWNS |
| NYJ | 38 | 176 | 214 | 11 |
| CAR | 77 | 178 | 255 | 19 |
| DEN | 111 | 255 | 366 | 21 |
| ATL | 149 | 242 | 391 | 25 |
The Bills have not faced the most exciting slate of opposing offenses this season, and as the offenses have gotten tougher, they've allowed progressively more yards on the ground and overall, and more first downs as well. That's not necessarily the best sign for their chances against elite offenses.
The good news is that they really only have four games against what one might call elite offenses – they play the Saints in Week 10, the Chiefs in Week 12, and the Patriots in Weeks 13 and 16. Other than that, the Bills face the Bengals, Buccaneers, a seemingly very stoppable Raiders team that may or may not be without Derek Carr, the Jets, the Chargers, the Colts, and the Dolphins twice. That's not exactly a murderer's row of offenses.
Their defensive performance so far has the Bills in first place, and this is the deepest into the season they've occupied that spot since the start of the Tom Brady-Bill Belichick era in New England. Whether they can maintain their position atop the AFC East or, assuming the Patriots eventually re-take control, in a wild card spot, will depend in large part on whether this start is real or a mirage. We've seen Buffalo jump out of the gates hot and fall off before. The pieces are all there for them to maintain if not quite this level of defense, then at least something close. If they can do it, they might make a surprise run at a playoff spot.