Arizona Cardinals rookie safety Tyrann Mathieu tore an ACL during Sunday's 30-10 win over the St. Louis Rams, ESPN reported Monday.
Mathieu also has a torn LCL, according to the NFL Network.
Coach Bruce Arians said after the game that Mathieu may have torn the ACL and was scheduled to have an MRI Monday.
Mathieu injured the knee while returning a free kick following a safety with about 9 minutes left in the third quarter. After getting tackled by Rams running back Isaiah Pead, Mathieu tried to run off the field but fell to the turf. He was carted to the locker room after being examined by team doctors.
Mathieu was replaced by Rashad Johnson, whom Mathieu replaced as starter after Johnson lost the tip of his left middle finger against the New Orleans Saints on Sept. 22.
Mathieu is second on the team with 64 tackles. He also has two interceptions and a forced fumble.