Zimmerman was seen by Dr. Bill Meyers, who is a specialist in this area, in Philadelphia on Friday, and the tear was diagnosed. The procedure is going to be done in Meyers' office Tuesday.
"If all goes well, it should be about a six-week rehabilitation until he's back and joins us again," said Nationals general manager Mike Rizzo.
Zimmerman was placed on the disabled list April 12, retroactive to April 10, with an abdominal strain. He's been working since then to get better, but it just wasn't happening.
"It's tough," Zimmerman said. "We went through a couple weeks to try and get it better, and it's frustrating. But it's nice to have, I guess, closure, to get it fixed and get working to come back."
Dr. Wiemi Douoguih, the Nationals' medical director, said that Zimmerman has an isolated tear of the rectus muscle in the abdomen.
Zimmerman had problems with his groin and his abdomen during Spring Training. He recovered from both before hurting the abdomen again in the first few games of the season, landing him on the disabled list.
"It started out very minor back in the beginning of March," said Douoguih. "We shut him down ... [but when] he slid into second base during a game the first or second week of April, then it really got much worse after that. That's when we shut him down and put him on the [DL]."
Zimmerman began working in rehab to try to improve the problem, but he kept running into setbacks.
"It just never got better," said head athletic trainer Lee Kuntz.
Zimmerman continued working, and both he and the team eventually decided to have the third baseman see Meyers in Philadelphia.
Zimmerman said surgery was something that could have been an option earlier, but they wanted to go for rehab first to see if that would work.
"Surgery's always the last thing; it's the last resort," Zimmerman said. "It's just one of those things where you have to go for it. You have to try and see if your body can heal itself. If it does, great. If not, then you go to surgery."
Missing Zimmerman has been painful for the Washington offense, which hasn't been able to score runs very well this season even though the Nationals are 9-8 without him.
"When you take your best hitter out of the lineup, you have to find different ways to score runs," Rizzo said. "Nobody's going to feel sorry for us. He's a huge part of our ballclub, but we're going to have to find ways to win games without him."
After Tuesday's procedure, Kuntz said Zimmerman will walk a mile the next day, and Douoguih said the baseball part of his recovery is going to depend upon how he feels.
"We did everything we could to try and avoid [surgery]," Zimmerman said. "It's just at a point now where it hasn't been getting better and to the point where we might as well go in there and fix it."
Jeff Seidel is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.