Kansas City head coach Andy Reid gave a preliminary report following the game and speculated that Charles had a torn ACL.
The Chiefs had a 17-3 lead over the Chicago Bears when Charles left the game, but squandered that lead and fell to the lowly Bears 18-17. Chicago scored two touchdowns in the final 3 minutes, 5 seconds to complete the rally.
Quarterback Alex Smith said after Charles left the game "production quit."
An MRI confirmed on Monday confirmed the torn ACL for Charles, which is the second of his career. He missed all but two games with the same injury in the 2011 season.
The running back did not put any weight on his leg as he was helped off the field, and was examined in the locker room by medical personnel.
Kansas City is off to a struggling 1-4 start, including losing four in a row, and will have to go the rest of the campaign without their starting tailback.