Mathieu exited Sunday's 40–17 win over the Philadelphia Eagles in the fourth quarter after sustaining the injury without much contract from an opponent. He underwent an MRI in Phoenix on Monday morning.
Mathieu, 23, previously tore the ACL and MCL in his left knee as a rookie in 2013 and missed the final three games of the season. He returned last year to play in 13 games with six starts.
The former LSU Tiger has 89 combined tackles and five interceptions on the year.
- Christopher Chavez