"The extent of the injury we don't know right now, but he couldn't finish the game," Bears coach Lovie Smith said soon after Chicago's 23-10 loss.
The Bears believe Forte suffered a high ankle sprain, a league source told ESPN NFL Insider Adam Schefter.
Forte suffered the injury early in the third quarter after catching a 14-yard pass and had the ankle retaped by the Bears' training staff. He briefly returned to the bench after receiving medical treatment but was quickly taken back to the locker room.
Michael Bush replaced Forte in the Bears' backfield and finished with 14 carries for 54 yards.
Before he left the game, Forte had 31 yards on seven carries and four catches for 49 yards.
Forte missed the final four games of the 2011 season with a knee injury but returned in time to participate in the Pro Bowl.
Designated as the Bears' franchise player after the expiration of his original rookie deal, Forte boycotted the entire offseason workout program but reported to camp on time after he signed a four-year contract on July 17.
The Bears don't practice until Monday in preparation for their game against the St. Louis Rams on Sept. 23.