PHOENIX -- It was an afternoon filled with new and uncertain beginnings Tuesday at Phoenix Municipal Stadium.

Oakland third baseman Scott Sizemore limped into the home clubhouse on crutches and in pain, but eager to embark on the road to recovery from an injury that will keep him off the field this season. The A's hunt for his replacement, a search that will take them inside and outside of the organization, has just begun.

On Monday, Sizemore tore the ACL in his left knee after his spike got caught in the grass while charging in on a bunt during drills. He expects to have season-ending surgery in three weeks when the swelling subsides.

"They don't know the extent of everything that is going to happen with post-surgery recovery, and they can't tell until they get in there," Sizemore said. "But the doctor said it was pretty straight forward and they know what they are expecting. It's nothing crazy, just the standard ACL recovery."

Sizemore's standard recovery will have an extraordinary impact on how the club will be constructed leading up to Opening Day. Josh Donaldson will get the opportunity to prove that he can be the club's full-time third baseman, but he's not the only one. Adam Rosales, Eric Sogard and Wes Timmons are also expected to get a chance to win the job.

The A's are considering all of their options.

"This is definitely an opportunity for [Donaldson] but at the same time, we definitely need to do our due diligence and see what else is out there on the market to make sure we can give Bob [Melvin] the best team that we can," Oakland assistant general manager David Forst said. "Typically, you don't see deals getting made the first week of March because teams spent all offseason putting a team together and they want to see what they have, at least for a little bit. It's not a time that most teams are really willing to engage, but they obviously know we have a need."

Sizemore, who hit .249 with 11 home runs and 52 RBIs in 93 games for the A's last season, appeared primed for a breakout year in 2012. He had positioned himself as the long-term solution at third base, but now that plan is on hold for at least a year.

"First and foremost, for him, I feel awful," Melvin said. "The first day he was here I told him I was thinking 90 RBIs and 85 runs. He said, 'I think you are a little short, Skip. Twenty and 100.' It was one of those years where we felt like we thought it would be a breakout year for him."

"I never had any knee issues, so that makes it that much worse that is was a freak accident," Sizemore said. "If I tried to do the same thing a million times, it would never happen again. For whatever reason it happened and you just have to deal with it."

Donaldson, the catcher-turned-third baseman, hit .261 with 17 home runs and 70 RBIs in 115 games for Triple-A Sacramento last season. He played third base in college and in the Dominican Republic this winter and will play the position exclusively for the remainder of Spring Training.

"He works hard and he's real athletic," A's catcher Kurt Suzuki said. "He's been taking grounders and he looks good over there at third. But what do I know? I'm not an infield coach, but from what I see with him being as athletic as he is, he seems to pick it up really fast."

Donaldson should expect a challenge from Timmons, a non-roster invitee. Timmons, 33, combined to hit .341 at Double-A and Triple-A last season.

"Timmons, if you look at his numbers, you are kind of puzzled why he has not gotten a shot at the big league level," Melvin said. "Those guys will get a lot of reps in games."

In the meantime, Sizemore's future "reps" will come during his rehabilitation sometime in the future.

"I worked my tail off to get in the best shape I possibly could to help this team win," Sizemore said. "It's devastating to not be on the field with the guys, going to war with them every day and helping them win. That's what I am going to miss the most, being a part of the team and group of guys working toward that common goal."