Brown, an All-ACC selection last season and again this preseason, injured his left knee during the team's scrimmage Monday night. An MRI confirmed the tear, the school announced Tuesday.
"Here is a young man who is an All-ACC and All-America-type player on the field, and even a better person off the field," coach David Cutcliffe said in a statement. "He has been and will continue to be an integral part of our program. He's as fine a representative of our program as we have, and nothing will change that. He's seen this type of adversity before, and I'm confident he'll overcome this challenge."
Brown has twice torn the ACL in his right knee during his Duke career, in 2010 and 2012. He returned to the field last season to start 13 games, finishing with 114 tackles, 11 tackles for loss and two interceptions. He ranked second in the ACC in tackles per game (8.77).
Beyond his on-field production, Brown was the unquestioned leader of Duke's defense and its biggest key toward taking a big step over its performance a season ago.
With Brown out, junior Deion Williams will get a first look at replacing him in the lineup. Brown would be eligible for a sixth season if he decides to apply for one.