OAKLAND -- The A's were off Thursday, but the front office obviously wasn't.

Acknowledging that right-hander Sean Gallagher has had virtually every day off since losing his bid for a spot in the starting rotation this spring and being moved into the bullpen as a long reliever, Oakland optioned Gallagher to Triple-A Sacramento.

A 23-year-old acquired from the Cubs in last summer's Rich Harden trade, Gallagher is expected to join a loaded River Cats rotation that features top prospect Vin Mazzaro -- a 22-year-old righty -- 23-year-old lefty Gio Gonzalez, 2007 first-round pick James Simmons -- a 22-year-old righty -- and veteran righty Edgar Gonzalez.

"Gallagher will start Saturday in Sacramento," A's manager Bob Geren told MLB.com via text message Thursday afternoon.

Gallagher, Chicago's 12th-round pick in the 2004 First-Year Player Draft, has appeared in three games for the A's this season, going 1-0 with an 8.10 ERA. His most recent appearance came April 15, when he allowed two runs on three hits over 1 1/3 innings against the Red Sox.

The key player in the Harden trade, Gallagher is 38-17 with a 2.77 ERA in 89 Minor League appearances, all but one of them coming as a starter. Thirteen of Gallagher's 34 big league appearances have come out of the bullpen, but all 11 of his outings for Oakland last season came as a starter -- when he went 2-3 with a 5.88 ERA while batting shoulder fatigue.

Asked if Thursday's move indicates a loss of faith in Gallagher's abilities, Oakland assistant general manager David Forst responded emphatically.

"Not at all," Forst said. "This just gives him a chance to go down, get regular work and hopefully get to where he can come back up and help us at some point."

Gallagher entered Spring Training as a favorite to claim one of several open rotation spots, and by the final week of camp it was clear that the final unclaimed spot would go to him or lefty Josh Outman. Gallagher, who posted a 5.12 ERA in seven spring outings, got roughed up in his final appearance in Arizona, so Outman, who had a 5.40 ERA during the spring but finished strong, got the nod.

The A's did not make a corresponding move to Gallagher's demotion, and while righty Chris Schroder had a 0.71 ERA in five appearances for Sacramento through Wednesday, it's not assured that Oakland will replace Gallagher with another reliever.

In the wake of Monday's rainout at Yankee Stadium, and with off-days Thursday and Monday, the A's temporarily moved Outman into the bullpen and won't need him to rejoin the rotation until May 2 in Seattle.

Thus, the A's might opt to insure themselves against more time missed by third baseman Eric Chavez (shoulder), utility infielder Nomar Garciaparra (calf tightness) and 38-year-old first baseman Jason Giambi (occasional leg fatigue) by bringing up a position player.

Among the top candidates at Sacramento is first baseman Sean Doolittle, who had a big spring with the big league club and was batting .294 with two doubles, a triple, two homers, eight RBIs and a .400 on-base percentage in 14 games through Wednesday.

Mazzaro has been the top starter at Sacramento thus far, posting a 1.93 ERA with a 1.14 WHIP (walks plus hits divided by innings pitched) in three starts.