Zimmermann had an X-ray and MRI to determine the severity of the injury, but manager Davey Johnson does not believe he will need to go on the disabled list.
"I'm really concerned about him," Johnson said. "I know he had some tests done today and they feel it's soft tissue that's the problem. It's something with rest that he could probably get by."
Johnson was unable to reach NL manager Bruce Bochy of the Giants to let him know about Zimmermann's status.
"I'm going to recommend to Bochy that he miss the All-Star Game," Johnson said before Friday's game at the Miami Marlins. "I was concerned with him last night. I knew that it was something that was still there. Jordan mentioned it before the game to (Steve) McCatty, the pitching coach."
Zimmermann, 12-4 with a 2.58 ERA in 19 starts, was named to his first All-Star team and is planning on making the trip to New York.
"I think it's great to go," Johnson said. "I haven't talked to our team doctor about it, but I think the rest would be something that's been lingering. He's having too good of a year not to use this time to rest."
Johnson plans on talking to Bochy about the possibility of having another Washington pitcher take Zimmermann's spot. Johnson mentioned Gio Gonzalez (7-3, 3.03 ERA), Stephen Strasburg (5-6, 2.45 ERA), and Rafael Soriano (1-1, 2.13 ERA, 24 saves) as potential replacements.