From the Lions' official site:
Today’s MRI revealed that TE Brandon Pettigrew’s knee injury suffered against the Green Bay Packers will require season-ending surgery.
This is really disappointing, because Pettigrew was really starting to play quite well lately. He and Stafford were developing a nice rapport, and Pettigrew was making a lot of big plays. Although Casey FitzSimmons and Will Heller did a good job of filling in for Pettigrew yesterday after he suffered the knee injury, it would have been nice for the rookie to continue to develop. Instead he is out for the season and will be headed to injured reserve.