Head Coach Les Miles says Mathieu was dismissed because he violated the rules of the school and the team.
But he didn't specify what rules were broken.
Miles simply says that Mathieu, who earned the nickname "Honey Badger," is a good person who has quote "behavioral issues."
He says he thinks the change might even be good for Mathieu.
"I really think that this could be a redirect that would really benefit him greatly. I think that it would be like me to see this is an opportunity for him to improve and to put himself in an even better position in the future. But, am I worried about him as being emotional about this time? Yeah, you know, I am."
Mathieu was beginning his junior year at L-S-U.
Local news station WDSU reports he is expected to transfer to another school and continue playing college football.
Mathieu, a finalist for the Heisman Trophy and the nation's top defensive player a year ago, played in 26 games for the Tigers during his two years, racking up 133 total tackles, 16.0 tackles for loss and four interceptions.
He also forced 11 fumbles, which ranks first in school history and seventh in NCAA history, to go with eight fumble recoveries.
Mathieu scored four touchdowns for the Tigers, two on punt returns and two on fumble returns.