Thomas sustained the injury during the second quarter of the Tigers' win over New Mexico State last Saturday. His knee twisted during a blitz and he had to be helped off the field before heading into the locker room on crutches.
LSU coach Les Miles has not yet confirmed the torn ACL.
Thomas, a sophomore, had 24 tackles, two sacks and an interception in five games this season. His 24 tackles are fifth-most on the team.
According to The Advocate, Thomas is expected to be replaced at nickelback by true freshman Jamal Adams, and Miles also named true freshman cornerback Ed Paris as someone who could play in Thomas' absence.
The Tigers are 4-1 this season, with their one loss coming at home to Mississippi State. They are ranked No. 15 in the AP Top 25 and Coaches Poll.
LSU hosts No. 5 Auburn on Saturday.
- Molly Geary