The extent of the injury to his left knee was confirmed by the team Monday. Bradford underwent an MRI exam Sunday night after the team returned from a 30-15 loss at Carolina.
Coach Jeff Fisher was to provide details at an afternoon news conference.
Bradford tore his anterior cruciate ligament in the fourth quarter when he when he landed on his knee after being shoved out of bounds by safety Mike Mitchell. Fisher said after the game that Bradford was in “significant pain” on the sideline.
Teammates had feared the worst after Bradford was carted off the field and on crutches in the locker room.
Bradford, the No. 1 pick in the 2010 draft, has thrown for 1,687 yards with 14 touchdowns and four interceptions this season. St. Louis had won its previous two games, with Bradford throwing three TD passes in each.
Bradford missed six games with a high left ankle sprain in 2011. He had season-ending shoulder surgery in 2009 when he was at Oklahoma.
Clemens made three starts in 2011 but hadn’t played this year until replacing Bradford against Carolina. He has made 12 career starts, seven of them with the New York Jets in 2007.
“It’s never easy when you see a teammate go down,” he said after the game. “When you see a person that you spend as much time with as I do with Sam and genuinely care about as a person. … I have great respect for what he does on the field, but just as genuine concern for him as a person.”
Added center Scott Wells: “Even if Sam wasn’t hurt, we still have some heavy lifting to do. But this adds extra to it.”