GLENDALE, Ariz. -- Cardinals rookie free safety Tyrann Mathieu "appears" to have an ACL injury, according to coach Bruce Arians, and will have an MRI on his left knee Monday morning, according to NFL sources.
Mathieu suffered a brutal hit in the third quarter of the Cardinals' 30-10 win over the St. Louis Rams.
Mathieu's injury "appears to be an ACL," Arians said. "We don't know for sure."
Mathieu was injured while returning a free kick with about 9 minutes left in 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.
Arians did not expand on the severity of Mathieu's injury.
"We'll wait and see," Arians said. "That's just an early prognosis of the exam that we have right now. He'll go through the MRI and all the rest of the tests, and just keep our fingers crossed."
Mathieu became a full-time starter in Week 4 against Tampa Bay and has exceeded expectations throughout the league after dropping to the third round in last April's draft.
Safety Rashad Johnson replaced Mathieu in the secondary in a reversal of fortunes. It was Johnson whom Mathieu replaced after Johnson lost the tip of his left middle finger in Week 3 at New Orleans.
Mathieu tweeted after going to the locker room:
I KNOW MY BROTHERS GOT MY BACK.. #Birdgang ! I'm with y'all every step of the way !
— Tyrann Mathieu (@Mathieu_Era) December 8, 2013
Mathieu's teammates are already thinking about the possibility of losing him for a length of time, potentially the rest of the season. The defense would lose Mathieu's 63 tackles -- second best on the team -- two interceptions and one forced fumble.
"It's more of his personality and his urge and his mentality on the field," Abraham said. "His physicality is good. Just not having him on the field. ... I'm used to talking to him even though he's a young guy, he prepares like a vet and he acts like a vet."
To Arians, the Cardinals could be losing the defensive rookie of the year.
"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]."