Nationals third baseman Ryan Zimmerman probably will miss the rest of the season because of an injured rib muscle on his right side that's sidelined him since last week.

Zimmerman was out of the lineup for Washington's game against the Philadelphia Phillies on Monday night, his fifth consecutive day off. After Monday, the last-place Nationals will have five games remaining.

"I'm not counting on him playing through the rest of the season," manager Jim Riggleman said before Monday's game. "If he were to say, 'You know what? I feel great. Let me take some batting practice.' ... I guess we would consider it, but I don't anticipate that happening."

Zimmerman himself didn't want to absolutely rule out the possibility that he could appear again in 2010, although he did say he only will play if he's pain-free.

"I wouldn't say it's all the way done yet," he said. "Obviously, I'm working every day to rehab and see if I can get back out there. But we're just being cautious. We don't want to come back and come back a little too soon and hurt it even worse."

He also noted that given "the circumstances that we're under" — the Nationals are assured of a fifth bottom-of-the-division finish in their six seasons in Washington — "there's really no reason to go out there or take pain medicine or do something like that to play. So for me to get out there and play, (it) would have to be 100 percent better with no risk to hurt it more."

Zimmerman, who turns 26 on Tuesday, leads the Nationals with a .307 batting average and is second on the team with a .388 on-base percentage, 25 homers and 85 RBIs. His previous best batting average for a full major league season was .292 in 2009.

His last homer came Aug. 30, and that was his only one after Aug. 15 this season, but he didn't blame his injury for that drought.

"I wouldn't say it affected it to the point where that's why I didn't hit a home run. I felt it a little bit, but no more than anyone else is feeling things like that," the 2009 NL All-Star said. "I was still getting my hits and being productive at the plate, but just hadn't hit any homers."

Zimmerman can't pinpoint a particular play on which he got hurt and attributed it mainly to wear and tear.

One reason to sit out the rest of the way: making sure the problem doesn't get any worse heading into the offseason.

"You don't want to go back and hurt it more and then spend your first month rehabbing an injury that you could have avoided," Zimmerman said. "I take a couple weeks off, but then after that, it's full go, getting ready for the next season. So it's a 3-, 3½-month process that a lot of us do to get ready for the next season and you wouldn't want to postpone the routine that I've gotten in, because the last couple years, I think what I've done in the offseason has been vital for the success that I've had on the field."