Montgomery, 24, injured his knee on a short 5-yard run in the fourth quarter of the Bears' 24-14 win over the Detroit Lions on Sunday. He had stayed on the ground, clutching his knee, and had to be helped off the field. He did not return to the game.
Prior to the injury, Montgomery had 106 rushing yards and two touchdowns, and he leads the Bears with 309 rushing yards and three touchdowns.
Damien Williams has been Montgomery's primary backup this season and figures to serve as Chicago's lead running back, if healthy, while Montgomery is sidelined.
Williams, 29, also suffered a thigh bruise this past Sunday against the Lions, and Bears coach Matt Nagy expects to have an update on the veteran running back's status as early as Wednesday.
ESPN's Jeff Dickerson contributed to this report.