Nationals third baseman Ryan Zimmerman will have surgery Tuesday and miss at least six more weeks. (STEVE NESIUS/REUTERS)

Washington Nationals third baseman Ryan Zimmerman will undergo surgery Tuesday to repair a torn abdominal muscle and “if all goes well,” he will return in six weeks, General Manager Mike Rizzo said. Zimmerman has already missed 17 games since he hurt himself sliding into second base April 9, an injury the team originally believed to be a severe strain.

Zimmerman has tried to rehab the injury, most recently playing catch with athletic trainer Lee Kuntz. Experiencing a dearth of progress, Zimmerman and the Nationals decided mutually for him to see specialist Bill Meyers in Philadelphia on Friday. Meyers diagnosed a tear of the rectus muscle — commonly known as the “six pack” — and it requires surgery, which will be performed Tuesday in Philadelphia.

“We did everything we could to try and avoid it, which is the first step, always,” Zimmerman said. “It’s to the point right now where it hasn’t been getting better, to the point where you might as well go in there and fix it and let this guy do it and get back to the road to coming back and helping the team win.”

Already missing their best pitcher, Stephen Strasburg, for the entire season as he rehabs from Tommy John surgery, the Nationals also will miss Zimmerman for, at best, about 60 games this season. The Nationals have gone 9-8 without Zimmerman, their undisputed best player, as veterans Jerry Hairston and Alex Cora have filled in.

“We see one of the best hitters in baseball that’s not going to be able to play for us for six weeks,” Rizzo said. “We’re scratching for runs and trying to win games. The guys that we’ve put out at third base have been terrific defensively and we’ve struggled to score runs. When you take your best hitter out of the lineup, you’re going to have to find different ways to score runs. Nobody’s going to feel sorry for us.”

“It’s frustrating,” right fielder Jayson Werth said. “It’s one of those things he’s been dealing with since spring training. Hindsight is 20/20. You could look at it like, he could have got it done then and we would have him now. But you can’t second-guess these things. It’s a tough injury to really gauge. Hopefully, he’ll get the surgery and be back soon.”

Zimmerman first missed three games early in spring training with a minor tweak he called “a non-factor” at the time. Later, he missed more than a week with a groin strain that was related to the abdominal strain, Nationals team physician Wiemi Douoguih said.

At that time, Douoguih said, Zimmerman underwent an MRI exam that showed “minor changes” from the previous issue. He returned in time for opening day and played the first eight games of the season with a minor strain. During the eighth, he slid headfirst into second base and immediately felt pain in his midsection.

“There’s a lot of guys who have little tiny strains and it doesn’t go on to anything more,” Douoguih said. “And it certainly appeared that that was going to be the case with him, and then he had a major traumatic event sliding into second base.”

Said Kuntz: “That one slide was the thing that tipped the scale.”

The Nationals believed Zimmerman had severely strained his abdominal muscle and immediately placed him on the disabled list. He rehabbed at Nationals Park while the team made an eight-day road trip, and he initially made progress. But his progress plateaued when he tried running and throwing. Zimmerman, even with hindsight, said he would not have proceeded differently.

“You never want to go right to surgery,” Zimmerman said. “Surgery is always the last thing. It’s the last resort every time. You want to rehab it, and we gave it that chance. We did all that stuff to a T, and it just wasn’t responding well enough, so we had to get it fixed.”

Zimmerman is on the disabled list for the second time in his career, but he also missed 20 games last season with various muscle pulls and strains, including an oblique injury that held him out the final two weeks.

The injury could complicate potential contract negotiations for Zimmerman, who is signed through 2013 but had expressed interest in a contract extension similar to those signed by Colorado Rockies shortstop Troy Tulowitzki and Milwaukee Brewers outfielder Ryan Braun.

The Nationals and Zimmerman’s representatives were expected to initiate discussions this season. Zimmerman, 26, remains one of the best players in the game, but his missing more than a third of the season will add a hurdle to determining his value.

June 14, six weeks from surgery , is when, if all goes well, Zimmerman could return. Including Saturday’s game against the Giants, he would miss 42 games in addition to the 17 he’s already missed, a grand total of 59.

“It’s tough,” Zimmerman said. “It’s good to see them playing well, though. It’s tough anytime you can’t play and you can’t help the guys you go to battle with, you go to spring training with. To see them play well, it makes it a little easier. So hopefully, they continue to do that. I know they will.”