From Rotoworld:

Paul Konerko is dealing with a mild concussion.
It was previously reported that he was sitting out Wednesday due to a sore back, but that's not the case. It's possible Konerko will require a trip to the 7-day disabled list. The veteran first baseman is batting a healthy .316/.388/.504 with 18 homers this season. He'll be reevaluated on Friday.