Rams defensive end Morgan Fox suffered a torn anterior cruciate ligament during organized team activities Thursday, a person with knowledge of the situation said.
Fox, 23, played in 16 games last season and had 2½ sacks. He also logged significant playing time in the Rams' playoff loss against the Atlanta Falcons after Michael Brockers suffered a knee injury.
The 6-foot-3, 275-pound Fox signed with the Rams in 2016 as an undrafted free agent after playing at Colorado State Pueblo.
Fox's injury depletes a defensive line that is already practicing without star tackle Aaron Donald, who has not reported for the voluntary workouts because of a contract dispute. Tackle Dominique Easley also is not participating in team drills while recovering from knee surgery.
The Rams allow media to attend OTA workouts only once a week. Reporters were not allowed to attend Tuesday or Thursday's workouts.
The Rams have seven remaining OTA workouts, which resume Tuesday.
The NFL Network first reported Fox's injury.