PHOENIX -- The last remaining piece of the 2009 Matt Holliday trade is making a lasting impression in camp.

Outfielder Shane Peterson, who came over from the Cardinals with the departed Clayton Mortensen and Brett Wallace in that deal, finished Tuesday batting .444 through 11 games.

Peterson has 12 hits in a team-high 27 at-bats, including a triple and a home run, and four RBIs. Surely he's making the A's offseason decision to protect him from the Rule 5 Draft and place him on the 40-man roster look like the right one.

"It's unbelievable," manager Bob Melvin said. "It seems like we're talking about him every day. It seems like he gets two good at-bats every day, and whether it's against lefties or righties, it doesn't matter. He hustles, runs hard, plays very hard."

On cue, Peterson picked up another two hits in Tuesday's 8-2 loss to the Royals, while also making his first start of the spring in left field. He had previously been used primarily in the later innings.

The 25-year-old, drafted by the Cardinals in the second round of the 2008 First-Year Player Draft, began 2012 at Double-A Midland for the second consecutive season, but finished it by hitting .389 in 38 games with Triple-A Sacramento. Overall, he finished with .326 average, nine home runs and 46 RBIs in just 86 games, after missing time at the beginning of the season with a sprained right ankle. He reached base safely via hit or walk in 78 of those games.

"We saw his numbers last year, didn't get to see him first-hand, but now we are," Melvin said, "and he's making big impressions."

Peterson can also play first base, and that added versatility could make him a prime target for a promotion this season should the A's need help at the big league level. Until then, he'll get his at-bats at Sacramento once camp concludes.

Weeks dealing with bone bruise in right shoulder

PHOENIX -- Turns out Scott Sizemore isn't the only A's second baseman ailing these days.

MLB.com learned Tuesday that Jemile Weeks is also dealing with an injury -- a bruised bone in his right shoulder he suffered while getting tangled up at second base on a double-play attempt in Friday's game against the Giants. He hasn't played since being injured.

Weeks, manager Bob Melvin said, is day to day and close to returning to game action.

The young second baseman was enjoying a productive spring before the injury, collecting hits in six of his first 11 at-bats -- including a home run -- for a .545 average, to go along with five RBIs in just four games.

Sizemore, meanwhile, could return to the lineup as soon as Thursday after being hit by a pitch in the left hand on Sunday. He took grounders on Monday and hit in the cage Tuesday morning, both signs of progression.

The two infielders are the top candidates to open the regular season as the starting second baseman.

Worth noting

• Though the A's are officially off on Wednesday, a couple of players will be in action at the Minor League facilities. Closer Grant Balfour will throw his second bullpen session since undergoing minor knee surgery at approximately 8:30 a.m. PT, while righty Jarrod Parker will make his second "start" of the spring at 9 a.m., when he's scheduled to face Minor Leaguers.

Parker is being sent to Papago Park so that the A's can keep all of their starters on the same schedule this spring.

• Looking to get back on track after giving up seven runs against the Giants in just one-third of an inning on Friday, lefty Travis Blackley did just that, firing two perfect innings with one strikeout against the Royals on Tuesday.