Sam Bradford had missed the previous three games due to a bone bruise sustained in Minnesota’s season opener.

Bradford was ineffective in his half of play against the Chicago Bears on Monday night and re-injured his knee before being replaced by Case Keenum for the rest of the game.

“He aggravated his injury when he got tackled on one of the plays,” head coach Mike Zimmer said of Bradford, via Courtney Cronin of ESPN.com. “I just thought it was best for him that we got him out of there. He didn’t want to come out. He wanted to stay in there and fight but I didn’t want to get him injured anymore.”

Bradford completed just 5 of 11 passes for 36 yards before Keenum replaced him late in the second quarter. Bradford was sacked four times in just 26 total snaps for the Vikings before leaving the game. One of the sacks also went for a safety to give the Bears an early 2-0 lead. The Vikings managed just three first down offensively with Bradford in the game.

Bradford was listed as questionable to play for Minnesota despite practicing throughout the week. It was clear early in the game that he didn’t have anywhere close to his typical mobility and wasn’t able to perform to his usual standard.

Keenum has played well in Bradford’s absence. He completed 17 of 21 passes for 140 yards and a touchdown in his half of work at quarterback for Minnesota.