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02/27/12 11:00 PM EST

Knee injury sidelines Sizemore for 2012

A's look to Donaldson as an 'intriguing' replacement at third

PHOENIX -- Prepared for the worst but hoping for the best, the A's were forced to face the undesirable of the two outcomes Monday evening, when they learned third baseman Scott Sizemore will miss the entire 2012 season with a torn ACL in his left knee.

Sizemore suffered the injury Saturday afternoon within two hours of the club's first full-squad workout, and on Monday morning he underwent an MRI administered by Dr. Douglas Freedberg that revealed the fateful news.

Due to swelling, the 27-year-old infielder will have to wait two weeks before being reevaluated to determine when he can undergo surgery. Meanwhile, the A's will turn to a catching prospect for help, with Josh Donaldson expected to get a long look at third base before the club decides whether to look outside the organization to fill the void.

Sizemore appeared primed for a breakout season with the A's, who acquired his services midseason last year via a trade with the Tigers. He immediately made the position switch from second base to third base to accommodate Oakland's need, and in the following months the A's watched him progress defensively and offensively.

At the plate, Sizemore hit .249 with 11 home runs and 52 RBIs in 93 games for the A's, and only Josh Willingham compiled a higher OPS than Sizemore's .778 mark.

"I really feel badly for Scott," manager Bob Melvin said in a release. "He's worked extremely hard, and was ready for a breakout season this year. We were counting on him to provide some much-needed power from the right side of the plate. Now, his total focus has to be on a successful surgery and rehabilitation, so he can return stronger than ever."

Melvin and Sizemore essentially arrived in Oakland at the same time. The third baseman was brought aboard to replace the demoted Kevin Kouzmanoff on June 6 last year, and Melvin's tenure with the A's began three days later, when Bob Geren was dismissed as manager. Coincidentally, Sizemore was responsible for Melvin's first win at the helm on June 11, when he provided the club's ninth-inning go-ahead run in Chicago to help put an end to a 10-game skid.

Finally, it seemed, the A's had found the answer to their long-term concerns at third base, which had turned into something of a revolving door since Eric Chavez's departure. In turn, Sizemore had found himself a fresh start following a disappointing time in Detroit, where he was once deemed the Tigers' second baseman of the future.

Monday's news only adds to a series of frustrating events in Sizemore's career. At the end of the 2009 season, he was handed the monumental task of replacing Detroit fan favorite Placido Polanco, only to watch the opportunity slip away when he broke his ankle in an Arizona Fall League game. The gritty Sizemore ultimately played through some pain the following spring and did well enough to land on the Opening Day roster as the Tigers' starting second baseman, as planned. But he struggled at the plate, hitting .205, before being sent down to Triple-A. Then, in 2011, he set out to prove himself once again in the months that preceded his move to Oakland.

Now, the A's plan to strip Donaldson of his catcher's gear in favor of full-time duty at third base this spring. Donaldson is no stranger to the position, having played it during most of his career at Auburn and every so often in the Minors. He also played at the hot corner exclusively in the Dominican this winter. The A's seem not to be so concerned with his defensive abilities, as much as they are about his potential to hit for the position.

Donaldson, 26, is a five-year Minor League veteran -- with a .268 average and .360 on-base percentage over that time. He most recently hit .261 with 17 home runs and 70 RBIs in 115 games for Triple-A Sacramento. Those numbers served Donaldson well as a catcher, but may not bode well as a third baseman. Moreover, he hit just .156 during a short Major League stint in 2010.

But Oakland is in no position to be picky, right now, as top third-base prospect Stephen Parker is still a year away from being Major League-ready. Donaldson will receive every chance to break camp as the everyday guy at third base.

"The Donaldson part is intriguing," Melvin said. "Certainly his bat has got some power, and he's been known to drive in a few runs."

"I'm looking for any opportunity, right now, to make this team and contribute," Donaldson said recently. "I believe in my abilities to play third, to be able to produce at the Major League level in the lineup. I just want to, hopefully, if I'm fortunate enough, prove myself to Bob and those guys that I deserve a shot. I'm hungry to play. I'm hungry to win a job."

Adam Rosales, Eric Sogard and Wes Timmons can also provide temporary help at the position. But they'll mostly be relied upon as utility players, given Oakland's lack of infield depth in camp.

Donaldson was seen getting work in at third base on Monday, and Melvin has already been encouraged by the results.

"So far, so good," Melvin said. "Everybody likes his actions. He worked double-time yesterday, and showed up early today. You can tell he's got a little different pep in his step, right now. Guys sense an opportunity, and I like to see that."

Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.