Lee was trying to tackle Bears running back Michael Bush when the top of his head hit Bush in the hip. Lee's neck compressed a bit and he went to the turf. He was able to walk off the field under his own power, which was a positive sign.
Trainers were taking a look at his neck on the sideline, so it appears Lee injured his neck and not his head. Considering Lee's history with concussions, that's a good sign for him. He's out for the game. He briefly returned to the game before being taken to the locker room.
Kyle Bosworth will take Lee's place in the lineup.
Lee's injury history is getting worse and worse as the seasons go on. He's been a productive linebacker when healthy, but that's becoming more and more rare.