Zimmermann, who is scheduled to participate in his second straight All-Star game on Tuesday, called the Nationals training staff out after throwing a pitch to Cameron Rupp with one out in the fourth. He showed no obvious discomfort, but left the mound after a short discussion with the training staff and Washington manager Matt Williams.
He walked off the field without a limp and jogged down the steps of the dugout, seemingly indicating the problem didn't involve his lower body. Zimmermann, who had Tommy John surgery in August 2009, left after allowing four runs and six hits. He threw 53 pitches, 36 for strikes.
Zimmermann (6-4, 2.79 ERA) has a 1.26 ERA since the beginning of June with 46 strikeouts and seven walks in 50 innings over seven starts.