"He's got a little issue under his shoulder blade. Had it last start," Nationals manager Matt Williams said after the game. "Could be an alignment issue, but as he finishes pitches, it grabs him every once in a while. Tonight it got a little bit worse. So we'll have to have the chiropractor look at him."
In the second inning, Strasburg winced after throwing a pitch and was visited by the team trainer, pitching coach Steve McCatty and Williams. The right-hander stayed in the game, but not for long.
Strasburg allowed two baserunners in the third before retiring Adeiny Hechavarria to end the inning.
"It's something that's just been lingering, and I just felt it more and more," he said. "I felt like it was something that I felt I could have gone through it, but at this point in the season you just want to get it right and not alter your mechanics too much."
Strasburg (2-3) left for a pinch hitter after allowing two runs and throwing 64 pitches in his shortest outing since a two-inning stint against Miami on Aug. 28, 2013.
Sammy Solis replaced Strasburg on the mound in the top of the fourth.
Information from The Associated Press was used in this report.