OAKLAND, Calif. — Oakland's Mike Sweeney will undergo arthroscopic surgery on his troublesome left knee Wednesday and is expected to miss four to six weeks.

The designated hitter and first baseman, who turns 35 next month, will have an operation performed by team orthopedist Dr. John Frazier to repair cartilage in his knee.

Sweeney had right knee surgery last year. After the cleanup procedure, he will spend a couple of weeks rehabilitating in San Diego with a therapist he's worked with before.

He hopes to be back to doing baseball activities in three or four weeks.

"It breaks my heart because I'm not going to be with the team I've come to love since spring training," Sweeney said today before Oakland played the series finale against the Los Angeles Angels. "We're so close to playing amazing baseball and getting to the top of the standings."

Yet Sweeney knew something was seriously wrong with his knee considering it hasn't responded to treatment. He was placed on the disabled list May 30 retroactive to a day earlier with inflammation in the knee.

Sweeney, who was batting .292 with two home runs and 12 RBIs in 40 games for the A's, had a cortisone shot Monday and then a gel shot Friday but neither helped alleviate the discomfort he felt.

"I knew from the beginning something wasn't right," he said. "The knee was locking up and catching. It felt like a piece of gravel was in there."

This is a blow for the A's, who are already without their other designated hitter, Frank Thomas. The Big Hurt turned 40 on May 27 and was injured that night against his former Toronto team stretching out a double. He has tendinitis above his right knee where it connects to the quadriceps muscle.

Oakland manager Bob Geren is hopeful the club will welcome back Thomas right after interleague play, when they won't need their DH much. Oakland hosts the New York Yankees for a three-game series Tuesday to Thursday, then plays three games across the bay in San Francisco followed by a three-game set at Arizona. After that, the A's have a nine-game homestand against the NL — Florida, Philadelphia and San Francisco.

An All-Star from 2000-03 and in 2005, Sweeney agreed to a minor league contract with the A's on Feb. 11 after spending his first 13 major league seasons with the Kansas City Royals — the team that drafted him. Then he went out and had a strong spring and made the A's opening day roster.

He has fought injuries in recent seasons and was limited to 74 games in 2007 because of his right knee and 60 the previous year because of his back.

Also Sunday, the A's activated right-hander Andrew Brown from the 15-day disabled list following his appendectomy last month and optioned left-hander Dallas Braden to Triple-A Sacramento.