Bell had struggled before the injury, completing just two of five passes for 10 yards. The play he was injured on was his only carry of the day.
Bell remained in the game for two plays before Sooners coaches noticed a problem and sent Knight into the game.
Bell, a junior, stayed on the sidelines for the rest of the game but he did not have his helmet with him.
It's been an up-and-down season for Bell, who played well against Tulsa, Notre Dame and TCU but has been inconsistent at other times.
Knight beat out Bell as the starter but struggled with his command of the passing game in the first two games.
Knight is the Sooners' fourth-leading rusher with 309 yards and a touchdown in five games.
Redshirt sophomore Kendal Thompson, who saw his first action late in the win over Iowa State, will back up Knight for the game. Freshman Cody Thomas, who is redshirting but has traveled with the team this season, is expected to be the third-team quarterback if Knight and Thompson were to be injured.