The Chicago Bears do-everything-running back Matt Forte left the Bears' 23-10 loss to the Green Bay Packers in the third quarter with a right ankle sprain.

Ian Rapport of NFL Network reported the running back has suffered a high ankle sprain, but the Bears do not have a clear timetable for his return.  Forte's availability will depend on his pain tolerance, the source said.

Second string running back Michael Bush becomes important for now as he becomes the starter if Forte is out for any length of time.

"That's why I'm here: They brought me here to pick up where (Forte) left off, if something went wrong," said Bush following Thursday's loss, according to the Chicago Tribune. "It's the insurance plan, I guess. I'll be ready."

Bush has been one of the premiere No. 2 running backs in the NFL. He ran for 977 yards in 2011, most after Oakland Raiders starter Darren McFadden was injured. Bush had 54 yards on 14 carries against the Packers.

With Bush taking on the role of the starter, he'll bring more of a power game to the backfield than the sure-handed, slick-stepping Forte. Though Bush has a different style, the Bears run game should be fine in Forte's absence.