Making his third start since being acquired from Cincinnati at the trade deadline, Mahle faced eight batters before manager Rocco Baldelli and a team trainer visited the mound in the third inning.
Mahle pitched 2 1/3 innings of hitless ball, walking one. He threw 29 of his 42 pitches for strikes, but his velocity was diminished.
Mahle is going to be further evaluated and the team is expected to provide an update later Wednesday or Thursday.
On July 6, the Reds placed Mahle on the 15-day injured list with a strained right shoulder. An MRI ruled out any serious issues.