Though his disabled list trips have been for a sore neck/upper back in late May and an oblique in early July, Strasburg's injury troubles actually began with a twisted left ankle during Spring Training. Attempting to pitch through it while having it heavily taped, Strasburg's April was below his normal standards (4.60 ERA, thanks in part to an unreasonable .402 BABIP), and his May was a complete mess. Five times Strasburg took the mound, and four times he didn't get through the fourth inning, getting knocked out by health concerns both on May 5 (3 2/3 innings) and May 29 (one inning).
Strasburg repeatedly said that the problems were more mechanical than physical, and the Statcast™ leaderboards back that up, because his four-seam fastball velocity of 95.76 mph is 38th of the 308 pitchers to throw at least 200 fastballs. Intentional or not, the various woes caused Strasburg to throw across his body, which in turn slightly affected his location and command. It appears that the biggest impact came when Strasburg was pitching with men on base, because check out the difference between his performance with and without men on as compared to his career marks:
Career, bases empty: .212/.265/.334 .599 OPS
Career, runners on: .252/.316/.385 .701 OPS
2015, bases empty: .211/.251/.372 .623 OPS
2015, with runners on: .323/.391/.458 .849 OPS
Like most pitchers, Strasburg has been a bit less effective over his career with runners on base, and in 2015, his bases-empty stats aren't much different than ever. But this year, suddenly, he's been getting crushed with runners on. Right there, that explains a big part of the ERA problem -- hits allowed in just the worst possible situations, the ones more likely to bring runs in. Clearly, that hadn't been an ongoing issue in the past, so it's fair to wonder whether Strasburg's mechanical and physical woes were exacerbated out of the set position due to the ankle and other issues.
Strasburg missed nearly a month and returned to throw three very impressive starts (two earned runs in 15 2/3 innings) before leaving the final one in the fourth inning with an oblique injury, which cost him nearly another month. It seems clear now, though, that the first trip to the sidelines were as much about fixing his mechanics as anything else, a journey that the oblique injury unfortunately interrupted.
For example, just look at how different Strasburg's curveball has been before and after that first DL trip:
Through all of this, Strasburg's year really has been better than it's looked. His Fielding Independent Pitching mark of 3.11 mark is tied with A.J. Burnett for 22nd (minimum 80 innings), and is better than big names like Sonny Gray, Jon Lester, or Felix Hernandez. FIP measures the things a pitcher can control -- strikeouts, walks and homers -- and Strasburg's underlying performance hasn't changed much. His year in terms of whiffs, walks and homers actually looks similar to his 2013, when he had a similar 3.21 FIP, but a much better 3.00 ERA.
But Strasburg's defense hasn't helped him much when he's needed it -- the generally porous Nats defense is 25th in Defensive Runs Saved at minus-23 was a big part of why Strasburg had a .355 BABIP in the first half, fifth highest in baseball -- and for a big chunk of the season, he needed it. But ever since that break to work out the mechanics, things have been different. His BABIP is just .213; his K/BB is 50/7 in 41 2/3 innings. It may be too late, because the Mets don't ever seem to lose anymore. But for Strasburg, it's back to business as usual. It's back to being an ace.