"We'll work through it [the severity of Montgomery's injury] in the next couple of days," Bears coach Matt Nagy said postgame.
The Bears don't believe Montgomery tore his ACL, but the team is conducting further testing Monday to determine the extent of the injury, a source told ESPN's Adam Schefter.
At the 11:10 mark of the fourth quarter, Montgomery ran off left guard for a five-yard gain but stayed on the ground and clutched his left knee in obvious pain after the play ended.
"As soon as he got tackled and I saw his facial expression and heard him, I was waving my hand for the trainers to come over as fast as they can," Bears receiver Darnell Mooney said.
Montgomery was eventually helped off the field and immediately taken back to the locker room, where the Bears ruled him out for the remainder of the afternoon.
"David's a leader," Bears quarterback Justin Fields said. "He's one of the team captains. He talks to us before every game. I mean, he's a dog. He's bringing the same energy every day at practice and every game. And I hope my boy, I hope it's not too serious. But you know, even if it is, he's still going to be a leader on this team and still going to continue to lead this offense each and every day."
Montgomery enjoyed a productive afternoon (106 rushing yards, two touchdowns) prior to the injury.
On the season, Montgomery leads the Bears with 309 rushing yards and three touchdowns.
In other injury news, Bears defensive lineman Akiem Hicks sustained a groin injury on Chicago's first defensive play of the game and did not return. Also, Montgomery's backup, veteran Damien Williams, suffered a quad injury.