The 49ers have had to make a change at quarterback, with Colin Kaepernick replacing starter Alex Smith.
The team has announced he suffered a concussion and will not return.
Smith was being checked by team doctors on the sideline, and just walked into the locker room with the medical staff.
Smith took a hard shot from Rams linebacker Jo-Lonn Dunbar, the apparent cause of the injury, but continued to play after that.