Nationals manager Matt Williams said during a radio interview that Zimmerman, Washington's primary third baseman for the past eight years, is dealing with an arthritic shoulder.
"We have to look at Ryan, though, and say, 'Would I like Ryan to play third? Yeah, I would. For a long time? Yes,'" Williams said Monday night in an interview with 106.7 The Fan. "But the fact of the matter is, Ryan's got an arthritic shoulder.
"It gives him problems, and sometimes it's tough on him. So we've started to do some things at first base with him, where we can give him a break sometimes."
Zimmerman underwent an MRI on the shoulder, but no structural issues were detected. The 29-year-old has not played a game at first base in his nine-plus years with Washington, but Williams thinks Zimmerman can make the transition.
"He enjoys playing first," Williams said. "We did a little bit at spring training, worked hard at the position, and he enjoys it over there.
"So it gives us options, it gives him an option to go play first when his shoulder's bothering him. ... So, do I see him moving to first? Yeah, from time to time. But I would like him to be our third baseman for a long time. We have to take into account his shoulder, too. And that's a concern. It's a concern of his; it's a concern of everybody else."
Zimmerman also is changing his throwing motion and practicing less to ease up on wear and tear.
He was out of the starting lineup again Tuesday night for the 5-0 win over the Marlins -- he hasn't played in the field since Saturday. He worked before the game on making sidearm throws from third, instead of his usual over-the-top delivery, and hopes to be back at his usual spot against Miami on Wednesday.
"We're going to try and do whatever it takes to make it feel better and, more importantly, make me be productive. That's the most important thing. I don't want to continue to hurt the team," Zimmerman said.
"We messed around with it a little today, and it felt a lot better throwing that way," he added. "I'll take some more ground balls tomorrow like that. The hope is to get into a routine where that becomes normal and effective and kind of go from there."
Williams also said Tuesday that he envisions using Zimmerman as a designated hitter in interleague games.
Zimmerman left Saturday's loss to Atlanta in the fifth inning because of soreness in his shoulder. He was used as a pinch hitter Sunday, and the team was off Monday.
He acknowledges he'll need to ease the load on his shoulder by cutting back on work between games.
"It's going to be hard for me. My whole career ... I've always been a guy who likes to hit a lot, likes to throw a lot, likes to take a lot of ground balls. It makes me feel prepared for the game," Zimmerman said. "Times have changed a little bit now. I'm going to have to adjust."
Zimmerman has already made two throwing errors this season, including a two-out misfire Saturday night that led to a pair of unearned runs in a 6-2 loss to the Atlanta Braves.
"Through spring and last year and things like that, it felt good sometimes, it felt bad sometimes," Zimmerman said Sunday. "But it's never really felt like this."
Zimmerman's shoulder has been bothering him for two years. He committed 12 throwing errors in 2012 and, after offseason shoulder surgery, committed 16 last year.
In other Nationals injury news, catcher Wilson Ramos says he has been told to expect to be sidelined up to two months after having surgery on his left wrist last week. He'll start rehab work next week, he said.
The Associated Press contributed to this report.