Keller exited Saturday's start against the Cubs with left side discomfort.
The 24-year-old was removed during the third inning after spiking a pitch. He's in line for the loss after allowing two runs on five hits with one strikeout and one walk over 2.2 innings. The severity of the injury remains unclear, so Keller should be considered questionable for his next turn through the rotation until the team updates his status.