Forte was hurt during the first quarter of Sunday's game against Kansas City when he was hit low by Chiefs linebacker Derrick Johnson. He was able to walk off the field before heading to the locker room.
Bears head coach Lovie Smith confirmed the sprain of the MCL, but didn't offer a timetable for Forte's return.
"We're still evaluating him," Smith said Monday. "He looked pretty good this morning. It is an MCL sprain and he didn't finish the game, so there is concern."
Forte entered Sunday's game leading the NFL with 1,475 yards from scrimmage and had run for 12 yards on five carries before leaving the contest. He leads the Bears with 997 rushing yards and is the club's top receiver with 52 catches out of the backfield.
It's the second major injury for a key member of the Chicago offense in the past three weeks. Quarterback Jay Cutler was lost with a broken thumb during a Week 11 game against San Diego. The Bears have lost two straight since to fall to 7-5.