Updated at 3:48 p.m.
Zimmerman, who has been on the disabled list for three weeks since aggravating the injury while sliding headfirst into second base, was not making progress in his attempt to return through rest and rehab. He consulted a second doctor in Philadelphia on Friday, at which point a torn rectus muscle (the six-pack in the abdomen) was diagnosed.
Surgery will take place Tuesday in Philadelphia, with specialist Bill Meyers performing. The Nationals said rehab will begin the day after surgery, and general manager Mike Rizzo said rehab should take six weeks "if all goes well."
That would put Zimmerman on target to return to the lineup in mid-June, about nine weeks removed from his last game.
"It's tough," said, who was hitting .357 in eight games before suffering the injury. "We went through it a couple of weeks trying to get it better. It's frustrating, but it's nice to have closure, get it fixed and get working to come back and help this team continue to play well like they have been."