San Francisco Giants third baseman Pablo Sandoval broke a bone in his right wrist and is expected to miss four to six weeks, adding to the concerns for the struggling World Series champions.

An MRI exam Saturday morning revealed the injury to the hamate bone, and Giants head athletic trainer Dave Groeschner said Sandoval probably will have surgery Tuesday in Arizona. Groeschner said the team isn't sure when, exactly, Sandoval got injured.

Sandoval's hand hurt while he was batting during San Francisco's 3-0 loss to the Washington Nationals on Friday night and he had an X-ray. The hamate bone is at the base of the hand, where the knob of the bat typically rests when a player is hitting.

"It's going to be quick. Four weeks, six weeks. I just want to get better as soon as possible," Sandoval said. "It's not a surprise. I told them I had pain."

Sandoval is hitting .313 with five homers and 14 RBIs this season.

"It's a tough blow for us," said manager Bruce Bochy, whose club had lost six of its previous eight games before beating the Nationals 2-1 Saturday.

"They all felt for Pablo," Bochy said of his other players. "They also know you have to focus forward."

The Giants already had infielder Mark DeRosa, outfielder Andres Torres and pitcher Barry Zito on the disabled list. They've also had their problems hitting of late, getting shut out in two of three games heading into Saturday and scoring a total of 21 runs in their last nine games.

Sandoval had been a bright spot, though.

"I feel for Pablo, because we've talked about, many times, how hard he's worked this past offseason to get himself in this kind of shape, condition, that he's in," Bochy added. "And of course, the way he was playing — it was really paying large dividends the way he worked out this offseason. Now he's got to get this taken care of."

A year ago, Sandoval dealt with significant weight gain and a loss of confidence. But he's dropped nearly 40 pounds, something that helped his range in the field.

Groeschner said that as he and Sandoval waited at a hospital Saturday, they talked about how the forced time away from the field will still allow the third baseman to work on his fitness.

"The one positive he's trying to take out of it is that he still wants to lose a few more pounds. And he's like, 'That's what I'm going to do. I'm going to work out.' He can work out his legs. He can still run," Groeschner said. "He's motivated to do that. So this is an opportunity to work on that part. He's going to have plenty of time, so that's what we're going to push."

Bochy said the Giants plan to call up third baseman Ryan Rohlinger from Triple-A Fresno on Sunday.

On Saturday, shortstop Miguel Tejada made his first start this season at third base — a position he's played regularly in the majors — and Mike Fontenot started at short (and was charged with a fielding error in the second inning when he dropped a throw). Bochy said Emmanuel Burriss also could get starts at shortstop.

Completing an unusual infield alignment for the Giants on Saturday was Bochy's use of starting catcher Buster Posey at first base, in place of Aubrey Huff. The reigning NL Rookie of the Year hadn't played at first since Aug. 22 at St. Louis, and Bochy said before the season he didn't plan to use Posey there at all — preferring to give his catcher a full day off whenever possible.

Posey did play first base regularly when he was called up last May before the Giants traded veteran catcher Bengie Molina to Texas.

Bochy actually had been thinking of putting Sandoval at first on Saturday, with Huff's batting average at .202.

"Huff needs a day," Bochy said. "He needs a break, mentally."