Ennis was injured Sept. 28 against Arkansas. Coach Kevin Sumlin said Ennis will have surgery to repair the torn anterior cruciate ligament in his left knee.
Suspended for the opener for violating team rules, Ennis appeared in four games and had six tackles. He started 11 games last season and had a career-high 23 tackles, including two for losses.
The Aggies (4-1, 1-1 SEC) play at Mississippi (3-2, 1-2) on Saturday.