Walston (elbow) isn't expected to return from the 15-day injured list until sometime in August, Theo Mackie of The Arizona Republic reports.

After turning in a 2.84 ERA, 1.42 WHIP and 11:9 K:BB in 12.2 innings through his first three big-league outings (two starts), Walston was placed on the 15-day injured list June 6. His injury was initially classified as left elbow inflammation, but the Diamondbacks have since listed him with a left elbow strain, according to MLB.com. While Walston has resumed throwing off flat ground from 60 to 100 feet, the Diamondbacks haven't yet indicated that he's been cleared for bullpen sessions, so he still looks to be several weeks away from being stretched back out for a starter's workload. All of Jordan Montgomery (knee), Eduardo Rodriguez (lat) and Merrill Kelly (shoulder) are further along in their throwing progressions and appear poised to beat Walston back from the injured list, so the 22-year-old southpaw will most likely be optioned to Triple-A Reno once he's activated.