Lions coach Jim Caldwell told reporters in Allen Park, Michigan, on Monday that the team will place Pettigrew on injured reserve.
Pettigrew had seven catches for 67 yards and a touchdown this season, and has been Detroit's primary blocking tight end.
With Pettigrew out for the season, the Lions have only two tight ends on the 53-man roster: Eric Ebron and Tim Wright. This isn't Pettigrew's first major left knee injury. His rookie season in 2009 ended on Thanksgiving after he tore the ACL in his left knee -- the same knee he hurt Sunday against the Rams, the story said.