SAN FRANCISCO — San Francisco outfielder Dave Roberts had surgery on his troublesome left knee today, and it could be 12 weeks before he rejoins the Giants.

Fred Lewis will play regularly in Roberts' spot in left field. Roberts batted .118 with one RBI in six games this season.

Team orthopedist Ken Akizuki performed the 45-minute procedure to clean out cartilage behind his kneecap caused by arthritis and wear and tear. Athletic trainer Dave Groeschner said there was not a tear, as the Giants believed there might be.

The 35-year-old Roberts will use crutches to get around for about 2½ weeks. He will begin light rehabilitation exercises Sunday to start regaining his range of motion and strength while the swelling from the surgery subsides.

If he progresses well in his recovery, Roberts could head out on a minor league rehab assignment in eight to 10 weeks, Groeschner said.

"We're going to go slow at first," Groeschner said. "He did have a significant amount of arthritis in there. We'll see how he responds after the first three weeks. We're definitely going to take it a little bit easy at first to make sure these things don't come back."

The knee had bothered Roberts since spring training, but this has to be a discouraging development considering he missed nearly a month in 2007 following left elbow surgery. Groeschner said he told Roberts this injury was more significant than a meniscus tear that would heal in about four to six weeks.

Also Saturday, the Giants recalled outfielder John Bowker from Triple-A Fresno and started him in right field against the Cardinals for his major league debut. To clear roster room, outfielder Clay Timpner was optioned to Fresno.