CHICAGO -- It turns out that Justin Morneau may have sustained more than just a sore left shoulder as a result of diving for a ground ball on Sunday.

While the Twins were missing Morneau's offense in Monday's shutout loss to the White Sox, the first baseman was working with trainer Rick McWane, taking a series of precautionary concussion tests after complaining of a headache. While the data came back mostly normal, McWane said the tests weren't entirely conclusive one way or the other.

"The test is broken down into a series of mini tests, testing different areas, and one of his tests came back with what the doctor described 'mildly below baseline,'" McWane said. "For the most part, the rest of the test looked good. So if you have to put a diagnosis on it, you can say he has mild concussion symptoms."

Given Morneau's history with concussions, which caused him to miss the entire second half of the 2010 season, the team is obviously watching this situation very carefully. Morneau will miss at least the rest of the series with the White Sox and will be tested again Wednesday or Thursday to monitor his progress.

"I'm sure it makes him nervous, but it's something that he's had to really deal with for a long time," manager Ron Gardenhire said. "It's something we don't take very lightly around here. We'll be really careful with this situation."

Morneau just came off the disabled list on Aug. 10 after missing two months due to a pinched nerve in his neck that required surgery. The headache and shoulder discomfort this time around, though, both stem from the second-inning play in Sunday's game, when Morneau made a diving attempt on Alex Avila's hit down the line. As he hit the ground, Morneau said his body just stopped instead of sliding, jarring both his shoulder and head.

He remained in the game and made a diving tag on a game-ending double play in the ninth inning, but he said he rolled through that dive and didn't experience any pain on that particular play.

"That's the only way I know how to play. I can't really look at that game and say I'd do anything differently," Morneau said. "I'm going to dive for that ball every time. It's just one of those plays that's just a reaction. If you're out there thinking, 'I'm going to dive' and then you try to stop yourself, you probably shouldn't be playing this game."

For now, the plan is to just rest Morneau for a couple of games and hope the headache completely subsides. The possibility of another stint on the disabled list hasn't been discussed at this point, and Morneau is planning on traveling with the team to Anaheim for its series with the Angels that starts on Friday.

"If it happens again, obviously we're going to have to look at it more," Morneau said. "But, right now, it just looks like it will only be a couple of days and I'll go back out there. We're just being cautious, which is good."

Mauer DH's, may catch on Wednesday

CHICAGO -- After missing six games with a sore neck, Joe Mauer returned to the lineup on Tuesday night as the Twins' designated hitter.

Manager Ron Gardenhire said ideally he would give Mauer one game at DH and then get him back behind the plate for Wednesday's series finale. That of course will depend on how Mauer feels after his first start since injuring the neck while lunging for a popup at first base on Aug. 23.

"Joe's ready to go," Gardenhire said. "He'll DH, and we'll see how all that goes, see how is body handles that and then catching [on Wednesday]. That's kind of the plan, so it'd be nice."

Gardenhire did backtrack slightly, however, when asked if first baseman Justin Morneau's absence from the lineup (mild concussion symptoms) could affect whether Mauer returns to the field behind the plate or at first base. Mauer, who was playing in just his 71st game on Tuesday, said the important part was just getting back in the lineup, regardless of what position he'd be playing.

"I still feel it, but basically it's to where I can get out there and play," Mauer said. "I've never had this [injury] before, so I don't know how long it will, but I'm assuming it will [linger]. It's just good to get back in there."

Mauer was 2-for-4 with two RBIs in the 8-6 loss to the White Sox, hitting a two-run double to right-center that temporarily put the Twins ahead 5-3 in the fifth.

"We all know what kind of hitter he is," Gardenhire said. "He's a good hitter, and he got in some situations where he put the bat on the ball. He wants to be out there playing. He's had some injuries, but it's nice to see him pick us up and hit the ball around."

Casilla pulled early from rehab start

CHICAGO -- Alexi Casilla exited a rehab start with Class A Fort Myers one out earlier than planned on Tuesday after experiencing discomfort in his right hamstring.

Casilla, scheduled to play five innings in the rehab start, was removed with two outs in the top of the fifth when he felt some tightness in the hamstring as he went for a ground ball up the middle. The second baseman injured his hamstring on July 27 and hasn't played with the Twins since reaggravating it in his first game back on Aug. 12.

Casilla will remain in Fort Myers to work with trainers while the team goes on a two-game road trip Tuesday and Wednesday. The Miracle will be home for a two-game set starting Thursday and then close out their season in Jupiter on Saturday and Sunday.

"He's going to stay there and be re-evaluated," Twins trainer Rick McWane said. "If he's able to play, we'll hopefully get him back out there this weekend and see how that goes."

Gordon, Bremer nominated for Frick Award

CHICAGO -- For the ninth straight year, fans will have a say in the names appearing on the ballot for the 2012 Ford C. Frick Award for excellence in baseball broadcasting.

The final ballot will contain 10 names, three of which will be decided by a fan vote, which includes Twins broadcasters Dick Bremer and John Gordon. Bremer and Gordon are two of 75 eligible broadcasters -- two representatives from each Major League team and 15 at-large nominations -- for the fan voting, running from Sept. 1-30.

Bremer, who covers the Twins for Fox Sports North, began covering the Twins in 1983, while Gordon has covered the team since 1987. Gordon, currently working for the Twins Radio Network, announced earlier this year that this will be his final season calling play-by-play for the Twins.

Online voting for fan selections for the 2012 Frick Award ballot will begin at 9 a.m. CT on Thursday, Sept. 1 at the Hall of Fame's Facebook site www.facebook.com/baseballhall and will conclude at 4 p.m. CT on Sept. 30. Results of the fan voting will be announced on Oct. 5.

Worth noting

• Ben Revere went 0-for-4 on Tuesday, snapping his nine-game hitting streak. Danny Valencia struck out as a pinch-hitter in the ninth to also end his nine-game hitting streak.

• Joe Mauer has multihit games in four of his last five starts, going 9-for-20 (.450).

• Minnesota starters have a 7.24 ERA over the past nine games.