With Pettigrew out for the season, the Lions have only two tight ends on the 53-man roster: Eric Ebron and Tim Wright.
Pettigrew had seven catches for 67 yards and a touchdown this season, and has been Detroit's primary blocking tight end.
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.
Pettigrew was the second draft pick of former general manager Martin Mayhew after No. 1 overall pick Matthew Stafford.