Stephen Strasburg didn’t come out to pitch the third inning of Friday night’s game between the Nationals and Braves in Atlanta after exhibiting a sudden velocity drop and apparent discomfort in the bottom of the second inning. The Washington Post‘s Adam Kilgore reports Strasburg left the game with a “muscular issue in his back,” though the Nationals have yet to issue an official statement.

After giving up a leadoff homer to Freddie Freeman in the bottom of the second inning, Strasburg’s fastball dropped from 98 mph in his strikeout of Even Gattis to 93-95 mph to the next two hitters, both of whom he retired. During those at-bats, Strasburg was seen rolling his shoulders and grimacing on the mound.

If Kilgore’s source is correct, this is not an arm injury. But it doesn’t mean it won’t require a disabled list stay, which would still be a painful blow to a Nationals rotation that just lost Ross Detwiler to an oblique strain.