Brown announced on Twitter that he suffered the injury during practice Monday.
Brown said he is not sure whether he will be able to play again, and he is beginning what he called "a long process with a lot of question marks."
Breaks my heart to announce this. Don't know why it happened to me... Going to try and keep my head up. Pray for me. pic.twitter.com/4777m4DydB— Dre Brown (@DreBrown27) April 20, 2016
The injury comes almost exactly a year after Brown tore his right ACL. He missed his freshman season.
Brown was expected to be a primary backup for sophomore Ke'Shawn Vaughn.
Team spokesman Kent Brown said coach Lovie Smith was not available for comment.
Earlier this month, Illini receiver Mike Dudek suffered his second ACL tear in a year.