CHICAGO -- Twins first baseman Justin Morneau has been diagnosed with mild concussion symptoms. The symptoms (headache, fogginess) developed Monday, while he was also dealing with a sore left shoulder.

Because of Morneau's history with concussions -- including last year's which cost him the final four months of the season -- the Twins had him take baseline concussion tests on Monday evening.

Based on the scores of the test, Morneau was diagnosed with mild concussion symptoms. He believes the symptoms stem from a play he tried to make at first base in the second inning Sunday, diving for Alex Avila's double down the line.

Morneau was upbeat today. The symptoms are subsiding, and he said it's "a reminder" of what he's been through. He'll be re-evaluated by a doctor on Wednesday but seems confident this will be a short-term thing.

He said his shoulder would have been good enough to play today.

"It's better today than it was yesterday," Morneau said of the concussion symptoms. "It's not even comparable to what it was [last year]. I think it's more of a precaution-type thing, but it's something we need to monitor. We'll hopefully be good to go in a couple days. We'll see the doctor again tomorrow and see what he says."

* Alexi Casilla's hamstring issue flared up again Monday, when he began his rehab stint with Class A Fort Myers. He'll take a few days, see how it feels, and resume playing for the Fort Myers team if all goes well.

* Manager Ron Gardenhire said he is giving Valencia a break because he's been playing a lot and looked sluggish on Monday. Luke Hughes is starting at third base.

* Joe Mauer is back in the lineup at DH and hopes to catch on Wednesday.

* Denard Span (migraine symptoms) has been feeling better during his hiatus and will return to Minnesota soon to begin working out at Target Field.

* The Twins had seven players from their organization chosen to compete in the Arizona Fall League: Scott Diamond, Aaron Hicks, Brian Dozier, Cole DeVries, Chris Herrmann, Bret Jacobson and Bruce Pugh.

Swarzak vs. Stewart

Anthony Swarzak will make his first start since July 18 on Tuesday night, when the Twins face the White Sox.

Swarzak was lined up to start last Friday, but the Twins needed him in long relief again Thursday because of Francisco Liriano's left shoulder injury.

With Liriano, Scott Baker and Nick Blackburn on the disabled list, Swarzak likely will get an extended chance as a starter now. He is 2-2 with a 3.16 ERA in five starts this season.

The White Sox counter with Zach Stewart, who came over from Toronto, along with reliever Jason Frasor, in the July trade that sent Edwin Jackson and Mark Teahen to the Blue Jays.

Stewart faced the Twins on Aug. 6 in his first start for Chicago, and held them to one run on eight hits over 6 1/3 innings, as the White Sox won 6-1. A righthander whose fastball averages about 91 mph, Stewart had one walk and two strikeouts in that start.

TWINS (56-78)

1. Ben Revere, CF
2. Trevor Plouffe, 2B
3. Joe Mauer, DH
4. Michael Cuddyer, 1B
5. Jason Kubel, RF
6. Luke Hughes, 3B
7. Rene Tosoni, LF
8. Rene Rivera, C
9. Tsuyoshi Nishioka, SS

Starting pitcher: RH Anthony Swarzak (3-4, 3.62 ERA)

WHITE SOX (67-65)

1. Juan Pierre, LF
2. Alexei Ramirez, SS
3. Paul Konerko, 1B
4. Alex Rios, CF
5. Dayan Viciedo, DH
6. Alejandro De Aza, RF
7. Tyler Flowers, C
8. Brent Morel, 3B
9. Gordon Beckham, 2B

Starting pitcher: RH Zach Stewart (1-2, 4.70 ERA)

U.S. Cellular Field. First pitch: 7:10 p.m. TV: FSN. Twins Radio Network