SAN FRANCISCO — The A's made it clear before this season that they were committed to developing their core of young players.

On Friday, they welcomed back one of those youngsters and shipped another to the minors.

Outfielder Ryan Sweeney was activated from the 15-day disabled list and started in right field against the San Francisco Giants. To clear a spot for him, Travis Buck got the bad news he was heading back to Triple-A Sacramento.

The move continues a turbulent season for Buck, a second-year player who had already been optioned to the minors once.

Buck was recalled from Sacramento on May 30 and started 11 games in right field. He hit three homers with seven RBI but batted just .171 in that span. He's hitting .160 with three homers and 13 RBI in 26 games overall.

Carlos Gonzalez has been impressive in center field, and Sweeney was ranked second among AL rookies in batting average (.293) and on-base percentage (.353) until going on the DL on May 30 with a bruised toe.

The A's want to take a long look at the two rookies playing together. And with the team beginning a six-game stretch in National League ballparks and no designated hitter, Jack Cust will be getting most of the at-bats in left field.

"I think Travis did a lot better when he came back up. His whole game looked better," A's manager Bob Geren said. "With interleague play, and Jack playing left field, we had to make a roster move

Sweeney had claimed the everyday center field job until fouling a ball off his left foot and landing on the DL. In his absence, Gonzalez has shown great range and a terrific arm in center. He also had nine RBI in his 12 games before Friday.

Geren said Sweeney would be his everyday right fielder.

"I don't care where I play as long as I'm still in the lineup," Sweeney said.

The decision means Emil Brown will probably only get at-bats against left-handed starters, though the Giants started lefty Barry Zito on Friday and the A's went with an all left-handed hitting outfield of Cust, Gonzalez and Sweeney.

Outfielder Rajai Davis also remains on the roster as the primary pinch runner off the bench and defensive replacement for Cust in the late innings.

'Hurt'-ful news

Frank Thomas' injured right quadriceps is more serious than originally thought, pushing back his return from the DL, Geren said.

Thomas visited a Los Angeles-area doctor Thursday, who determined the slugger needs significantly more recovery time. Thomas has been out since May 27 with tendinitis in his quad, but the A's were hoping he'd be ready for a home series with Florida that begins June 20.

"He won't do any baseball activity until at least July 1," Geren said. "The doctor felt he was going to need that much time to heal. Nothing has changed (in his diagnosis) other than the healing process is going to take longer."

Thomas will remain home in Las Vegas rehabbing for the time being.

Combined with the absence of Mike Sweeney — who had arthroscopic surgery on both knees Wednesday — the A's will be without two of their most potent right-handed bats for a prolonged period.

Mike Sweeney is hopeful of returning after the All-Star break.

Short hops

The A's likely will be making another move soon, as Santiago Casilla (strained elbow ligament) appears ready to come off the DL. He worked a scoreless inning Friday for Sacramento against Tacoma, allowing one hit with a walk and a strikeout. "... Fellow reliever Joey Devine (elbow) said he'll start playing catch today. "... Count Geren among those in support of instant replay for home run calls, in light of a report that replay could be instituted before season's end. "I think with so many new ballparks, where fans are so close, I think it's great," he said.

— Joe Stiglich