DETROIT -- When right-hander Sean Gallagher was activated from the disabled list Wednesday to start against the Tigers, he became the 15th rookie on Oakland's active roster of 30 players.

A season-high seven of them were in the starting lineup for the finale of a three-game series at Comerica Park, but before the game it was announced that another one of the rookies won't be starting anytime soon.

Lefty Gio Gonzalez, winless in his past five starts, is being sent to the bullpen and replaced in the rotation by fellow rookie Josh Outman, who will make his first big league start Saturday in Oakland against the Rangers.

Outman has made two relief appearances since being called up from Triple-A Sacramento on Sept. 2. He was scheduled to start the second game of Oakland's doubleheader in Baltimore last Saturday before the first game was rained out.

"I just think that pitching in a few non-pressure situations might be the best for him right now," A's manager Bob Geren said of Gonzalez, who on Monday had his worst outing since being called up from Sacramento on Aug. 6.

After being given a 2-0 lead before taking the mound in Detroit, Gonzalez gave up three runs on two homers in the bottom of the first, and he fell apart in the second, allowing six runs -- including a grand slam by Gary Sheffield -- before being yanked with two outs in the inning.

Gonzalez's main problem has been poor command. All four of the homers he's allowed in his past 5 2/3 innings of work have been on pitches up in the zone, and he's walked or hit 21 batters over his past five starts (17 innings).

Geren suspects that Gonzalez's confidence has been flagging, and the move is being made in part to reverse the trend.

"A lot of the game is mental, and this is the highest level of the game," Geren said. "So the best and the toughest mentally are the ones that succeed. ... I just try to keep everybody in a position where they have the best chance to succeed.

"I look at all of these guys as long-term players and do what's best for the organization."

Geren said the green and gold will get greener still when Triple-A Sacramento's season comes to an end.

The River Cats won the opener of the best-of-five Pacific Coast Championship Series on Tuesday night, which concludes Sept. 14. If Sacramento wins the PCL title, it will have a one-game playoff for the overall Triple-A title in Oklahoma City -- "The Bricktown Showdown" against the International League champs on Sept. 16.

Among the candidates for promotion are 22-year-old outfielder Carlos Gonzalez, who was with the A's from May 31 to Aug. 28, and 21-year-old righty reliever Henry Rodriguez, who wowed the Yankee Stadium crowd during the 2008 XM All-Star Futures Game with radar-gun readings of 100 mph.

Geren said the A's have already decided which players they'll call up after the Triple-A playoffs, but he didn't want to release the names.

Asked if it would be "three or four" players, Geren said, "or less."