Pettis, a third-round draft pick out of Boise State, has made three starts while appearing in 12 games and has 27 catches for 256 yards. With their receiving corps hit hard by injuries, Pettis has been the Rams' No. 3 receiver during most of the second half of the season.
Pettis will be eligible to return after the Rams' second game next season, and he can participate in offseason and preseason practices and games.
The Associated Press contributed to this report.