Minnesota Vikings linebacker Desmond Bishop tore his right ACL during Sunday's loss to the Carolina Panthers.
Coach Leslie Frazier said Monday that Bishop will need season-ending surgery.
Bishop missed all of 2012 because of a hamstring injury while with the Green Bay Packers.
The Packers released Bishop in June, and he signed a one-year deal with the Vikings. He took over as team's weak-side linebacker from Marvin Mitchell on Sunday, but his role was short-lived.
"As we gave him more opportunities, he had shown that he was ready to get a starting assignment," Frazier said. "I wanted him to solidify it through his play in that ballgame yesterday.
"Unfortunately, he got injured, but he was playing well up to that injury."
Mitchell could get his job back or rookie Gerald Hodges could win it.