Zimmermann, 28, has been one of the best pitchers in the National League this season, posting a 3.03 ERA with 101 strikeouts and 20 walks over 113 innings.
The injury will keep Zimmermann out of the All-Star game, which is being held on Tuesday at Target Field in Minneapolis, Minn.
Zimmermann talked about the injury on Friday in The Washington Post:
“I don’t think it’s anything major, to be honest with you,” Zimmermann said. “It’s a little cramping, and it just felt really tight. So I didn’t want to push it.”
Zimmermann will undergo an MRI on Saturday.
The Nationals opened Saturday in first place in the National League East. The team has two games remaining against Philadelphia before the All-Star break.