HOUSTON -- The inevitable became official Tuesday as the Giants announced that infielder-outfielder Mark DeRosa will undergo season-ending surgery on his ailing left wrist.

A date for the procedure has not been scheduled, since Dr. Thomas Graham, the hand specialist who will perform DeRosa's surgery, is moving from Baltimore to the Cleveland Clinic. But Giants head athletic trainer Dave Groeschner said that DeRosa would be shelved for the year, even without the slight delay.

"We'd just be trying to push it too much," Groeschner said. "There's just too much at stake."

A prognosis, Groeschner added, will have to wait until after the surgery.

DeRosa, one of the most hotly pursued free agents last offseason, signed a two-year, $12 million contract with the Giants and was the team's Opening Day left fielder. But he admitted recently that he never felt quite right after undergoing wrist surgery last October for an injury he sustained in midseason. Batting .194 in 26 games with the Giants confirmed that something was wrong.

DeRosa hadn't played for the Giants since early May and knew that he would need surgery sooner or later. It turned out to be sooner, though he went on a Minor League injury rehabilitation stint in the hope that a preceding rest would help calm down his wrist. It didn't.

Graham is expected to fix the tendon sheath in DeRosa's wrist and might release the carpal tunnel, which would free the nerve that caused numbness in the 35-year-old's ring and pinky fingers.

Posey takes break against Houston

HOUSTON -- Given the steady activity Buster Posey has received with the Giants, his absence from Tuesday night's lineup was noteworthy.

Posey, batting .303 overall but mired in a 2-for-27 skid spanning his previous seven games, had started 20 of 21 games since being recalled from Triple-A Fresno on May 29. Manager Bruce Bochy figured that Posey could use an extra break combined with Monday's scheduled off-day. Besides, resting Posey enabled Bochy to start shortstop Edgar Renteria, who began the game batting .310 (9-for-29) with two home runs and seven RBIs against Astros right-hander Roy Oswalt.

With Renteria playing, Juan Uribe moved to third base and Pablo Sandoval filled Posey's void at first.

Posey's slump didn't seem to affect his usual calm demeanor.

"I think the most important thing is to stay with the same approach and not change anything," Posey said after taking early batting practice. "Mechanically, I feel fine."

Bochy's biggest challenge remains finding time for Posey to catch. Posey has started at catcher once as a Giant while playing primarily first base.

"I'd like to get him behind the plate occasionally so he doesn't get too rusty there," Bochy said.

Even when Posey doesn't replace Bengie Molina or Eli Whiteside in the lineup, he can maintain a semblance of familiarity with catching through various drills and exercises. Tuesday, for example, he caught Matt Cain's between-starts bullpen session.

Martinez penciled in to start vs. Red Sox

HOUSTON -- Manager Bruce Bochy reaffirmed that Joe Martinez is slated to fill the fifth starter's spot Saturday against the Boston Red Sox.

"Right now," Bochy said, allowing himself some wiggle room.

Martinez likely would be scratched if he's needed to pitch long relief sometime within the next few days. That would prompt a clamor among fans for Madison Bumgarner, who's 7-1 with a 3.16 ERA in 14 starts for Triple-A Fresno.

What's certain is that Todd Wellemeyer won't be returning right away. Bochy said that Wellemeyer, the Giants' No. 5 starter until he strained his right quadriceps on June 10 at Cincinnati and went on the 15-day disabled list, needs at least two more weeks to recover. Wellemeyer is rehabilitating his injury at the Giants' training complex in Arizona.