BALTIMORE -- The A's made several moves before Monday's series opener with the Orioles, most designed to try to improve their problems at third base.

They reinstated infielder Adam Rosales from the 60-day disabled list and recalled infielder Scott Sizemore from Triple-A Sacramento. Oakland optioned struggling third baseman Kevin Kouzmanoff and pitcher Fautino De Los Santos back to Sacramento.

Kouzmanoff (.221 average, nine errors) and Andy LaRoche have combined for most of this year's starts at third base. The A's designated LaRoche for assignment on Sunday.

"The production from that spot has not been there either offensively or defensively," said manager Bob Geren. "Now we're going to give somebody else a chance to produce in that spot."

The 29-year old Kouzmanoff hasn't been in the Minors since 2006, but has struggled throughout the season. He's hoping to get himself straightened out at Sacramento, but when asked his thoughts on this move, it's easy to see the veteran got caught off guard.

"Surprise, shock. I didn't see it coming," Kouzmanoff said. "As everybody well knows, I started off slow on both sides of the ball. I was unsatisfied with my play so far this season. I got the news that I'm going down, and all I can do is just go down and get my reps in and keep working and try to work my way back."

Rosales and Sizemore will share duties at third. For Monday's game, Rosales got the start, batting seventh. He had been on the disabled list after missing all of Spring Training with a fractured right foot.

Rosales hit .271 with seven homers and 31 RBIs in 80 games with the A's in 2010.

The A's acquired Sizemore from the Tigers on May 27 in a deal for pitcher David Purcey. He was hitting .267 in nine games at Sacramento, playing most of his games at third base, but Geren said both Rosales and Sizemore can help Oakland at third and in other spots.

Geren said that they will both be given opportunities at third. But Sizemore also can play second while Rosales plays second and short, and both could even be in the lineup at once, giving Cliff Pennington and Mark Ellis some breaks.

De Los Santos made his Major League debut Saturday in Boston. He struck out Jed Lowrie, the only batter he faced.

On Sunday, De Los Santos gave up one run on two hits in two innings and struck out four. Geren said he showed a "good live arm."

Weeks reportedly close to getting called up

BALTIMORE -- According to a report on CSNBayArea.com on Monday night, the A's will soon call up second-base prospect Jemile Weeks from Triple-A Sacramento.

The A's might need some second-base help after starter Mark Ellis strained his right hamstring in Monday's 4-2 loss to the Orioles. Ellis said after Monday's game that he'll have a better idea on Tuesday about the severity of the injury.

The 24-year-old Weeks, who was pulled out of Sacramento's game in the second inning Monday, was Oakland's first-round pick in the 2008 First-Year Player Draft and is the younger brother of Milwaukee second baseman Rickie Weeks.

Weeks is hitting .321 with three homers and 22 RBIs in 44 games this year. In addition, Weeks has a .414 on-base percentage.

The A's called up Scott Sizemore on Monday, and he can play second as well as third. Sizemore came on for Ellis in the sixth and went 1-for-2 in his first game with Oakland.