Tyrann Mathieu has torn ACL

— -- Arizona Cardinals rookie safety Tyrann Mathieu tore his left ACL during the team's 30-10 win Sunday over the St. Louis Rams, a league source told ESPN NFL Insider Adam Schefter.

NFL Network reported Monday that Mathieu also tore his LCL.

Cardinals coach Bruce Arians said Sunday that it appeared Mathieu had injured his ACL. Mathieu was scheduled to have an MRI exam Monday.

Mathieu was injured while returning a free kick midway through the third quarter following a safety by teammate John Abraham. After being tackled by Rams running back Isaiah Pead, Mathieu began running off the field when he fell to the turf. Team doctors came to his aid, and he was eventually carted off the field.

Mathieu became a full-time starter in Week 4 against Tampa Bay and has exceeded expectations after having dropped to the third round in April's draft.

Safety Rashad Johnson replaced Mathieu in the secondary on Sunday in a reversal of fortunes. It was Johnson that Mathieu replaced after Johnson lost the tip of his left middle finger in Week 3 at New Orleans.

Mathieu has 63 tackles -- second-best on the team -- along with two interceptions and a forced fumble.

On Sunday, Arians said he believed Mathieu was on pace to earn Defensive Rookie of the Year honors.

"To me, he's a lock," Arians said. "I don't know if anybody else has played as much as he's played and all the positions he's played. I just wish that he [Rams punter Johnny Hekker] kicked that damn ball to Patrick [Peterson]."

ESPN.com Cardinals reporter Josh Weinfuss contributed to this report.