PHOENIX -- Between shortstop and second base, the A's worked out enough players to field a full nine-man squad during Monday's workouts.

Five took grounders at shortstop, while four fielded their own next door at second, a scene that perfectly embodied the type of competition crowding the middle infield in camp this year.

For the time being, Oakland's staff is going with the assumption that newcomer Hiroyuki Nakajima can handle everyday duties at shortstop until proven otherwise. But the Japanese infielder's double-play partner has yet to be identified -- and likely won't be for weeks.

In the meantime, don't expect manager Bob Melvin to tip his hand, not out of secrecy, but because even he isn't sure how the situation will pan out. Evaluations of the contenders, led by Scott Sizemore and Jemile Weeks, will be ongoing.

"I think we're pretty transparent in what we're telling you about who's playing when and where," Melvin said. "It really is going to take a good portion of spring and games to figure out who plays where and what the formula's going to be during the season."

Second base was Weeks' position to lose at this time last year, following an outstanding rookie campaign. The 26-year-old entered the 2012 season as the everyday guy, as expected, but was soon swimming in a stream of struggles and, in August, was demoted to Triple-A Sacramento.

After batting .303 with 22 stolen bases and a .761 OPS in 97 games as a rookie in 2011, Weeks hit just .221 in 118 games last year. Moreover, his batting average on balls in play tapped out at .254, remarkably low for a player with so much speed.

"He just got into a little bit of a hole there early on, and sometimes it's tough to find your way out of it," Melvin said. "You know, you talk about that sophomore slump, and it kind of happened to him last year."

While Weeks was suiting up for the River Cats, Josh Donaldson was making his mark at third base in Oakland, a turn of events that led the A's to make the offseason decision to move Sizemore -- held out of the 2012 season while recovering from knee surgery -- from third base back to his natural position at second base.

Sizemore, having compiled 52 RBIs in just 93 games for the A's in 2011, was primed for big things in what turned out to be a lost '12 season. Those expectations haven't changed going into '13.

"It's been a difficult year for him," Melvin said. "I know that it can be a very lonely process. He had big plans last year, based on what he did in little more than half a year for us the year before. He had finally settled in, after learning the position of third base, and all of a sudden, that happens to him and you have to rehab. But he's put himself into a position, physically, where he's ready to go."

Where he goes is unclear. Should Donaldson stumble this spring, the A's could easily transition Sizemore back to third base, seemingly clearing the way for Weeks to call second base his own again. Maybe that would have been a possibility two weeks ago, but Jed Lowrie has since joined the mix, further adding to the competition at second.

Lowrie has been pegged by the A's as a rover of sorts, given his ability to field all four infield positions. But, by the time Cactus League action closes, he could potentially stand out as their best option at second base.

Assuming Donaldson lands at third base and Nakajima is still manning shortstop with Brandon Moss at first base, that would only leave room for two roster spots between Weeks, Sizemore and fellow infield candidates Adam Rosales, Eric Sogard and Grant Green. The ultra-versatile Rosales, out of options, is likely to get one, and Sizemore could potentially edge out Weeks for the other one in this scenario simply because he can also play third.

Melvin knows Weeks wants to prove himself to be better than an odd man out, though.

"He has the ability to do what he did the year before," Melvin said, "and I know he's confident in his abilities and looking forward to returning to that type of player."