An MRI on the left knee of Bears starting right guard Lance Louis revealed a torn ACL ligament, ending the season of the team's most consistent offensive lineman.
The Bears announced that they have placed Louis on the Injured Reserve.
On the opening drive of the third quarter, Bears quarterback Jay Cutler was off mark on a pass to open receiver Brandon Marshall, who tipped the ball into the air. Minnesota Vikings cornerback Antoine Winfield intercepted it and sprinted down the left sideline.
Louis was turning toward Winfield, who passed him, and he was blindsided by Vikings defensive end Jared Allen. As he crumbled to the ground, Louis' left leg appeared to get caught up in the turf.
"I thought it was a legal hit. He's running to make a tackle, so I just made a block," Allen said. "I turned around, he was trying to make a tackle, and I threw myself into him. Just trying to make a block, just like we're taught.
"I'm never trying to hurt anybody. I'm trying to make a block and spring our guy. It's unfortunate."
Louis has started all 11 games this season, and he started 13 of the 14 he played in 2011. A seventh-round pick 2009, Louis is a converted tight end who has emerged as arguably the team's best offensive lineman.
The website Pro Football Focus has him rated as the team's best lineman, giving up two sacks, two hits and seven hurries. By contrast, right tackle Gabe Carimi has given up seven sacks, nine hits and 28 hurries.
"Lance has proven to be one of the top guards in the NFL and it is disappointing that his season has come to an end," agent Bruce Tollner said in a statement. "Lance will have a quick recovery and be back to playing at an elite level."
Carimi replaced Louis in the lineup and did a solid job. But the 2011 first-round pick had never played the position before Sunday's victory over the Vikings. Another option is Edwin Williams, a college center who has started at guard for the Bears.
The most obvious candidate would have been Chilo Rachal, who started eight games this season after signing as an unrestricted free agent. But Rachal left the team after learning that he was being benched last week, and the Bears placed him on the reserve/ non-football injury list, effectively ending his tenure with the club.
Last week, the Bears signed guard Chris Riley, who played at Illinois State, to the practice squad and promoted James Brown to the 53-man roster, to take Rachal's spot. Brown played left tackle at Troy and right tackle in high school.