PHOENIX - Oakland Athletics outfielder Bobby Kielty needs arthroscopic surgery on his left knee and will be sidelined for three-to-six weeks.

Oakland Athletics outfielder Bobby Kielty steps out of the cage while taking batting practice before the A's play the Colorado Rockies in an interleague baseball game in Denver in this, June 19, 2006 file photo. Athletics outfielder Bobby Kielty needs arthroscopic surgery on his left knee and will be sidelined for three-to-six weeks. (AP Photo/David Zalubowski)

Kielty, who came to camp in the best shape of his career, injured the knee Sunday during a rundown drill — and just when he was ready to get through a healthy spring training, for a change.

"It's definitely a bummer," Kielty said Wednesday. "Right now I could play on it, but we're deciding that the best thing is to make sure it's good for the season."

If he'd been hurt at the end of March closer to opening day rather than the end of February, Kielty said he would have opted against having the scope. Manager Bob Geren said the injury wouldn't affect Kielty's chances of making the team, though it keep him from contributing right away once the season starts.

"We don't know how long it will set him back," Geren said. "We don't judge a huge amount guys who have major league time."

Kielty agreed to a $2.1 million, one-year contract in late January that avoided arbitration. He will likely be primarily a backup outfielder again for the A's. Kielty, who is strongest against left-handed pitchers, batted .270 with eight home runs and 36 RBIs in 81 games for the AL West champion A's last season. He has dealt with strained side muscles during spring camp in recent years.

"I'm going to try to get back as quickly as possible and be ready to go," he said. "It's pretty frustrating because I really worked hard this offseason to get in shape. I feel I'm in the best shape I've been in for baseball. This is just another setback."

Center fielder Mark Kotsay also is out and underwent an MRI exam Wednesday. He has a stiff back, a problem that affected him several times in 2006. He called the recurring back problem very discouraging.

"But you can only control what you can control," said Kotsay, who played in only 129 games last season.

Milton Bradley will start in center field in place of Kotsay for Oakland's first two Cactus League games, beginning with Thursday's exhibition opener against the Milwaukee Brewers.

"He's excited about the opportunity," Geren said. "I told him in the winter that if anything happens to Mark or if Mark needed a day off, he was my choice in center."

Shortstop Bobby Crosby, the 2004 AL Rookie of the Year who's been out since late August with a back injury, hasn't been slated to play yet.

He is still waiting to face live pitching for the first time.

Gold Glove third baseman Eric Chavez, who nursed tendinitis in his elbows and forearms for most of last season, also will sit for Thursday's game and is slated to play for the first time Friday — also against the Brewers. New designated hitter Mike Piazza will play Friday, too.

Geren will have a good chance to see some of the team's young talent.

"I want to see all of them," he said.