GLENDALE, Ariz. --
remarkable rookie season appears to have come to an end prematurely.
Mathieu suffered what
coach Bruce Arians said appears to be an ACL injury while returning a free kick after a safety in the third quarter of Arizona's 30-10 victory over St. Louis on Sunday at University of Phoenix Stadium.
Mathieu was tackled by the
, and his left leg got twisted on the play. As he tried to run off the field, he fell down. He was later taken off on a cart.
Mathieu will have an MRI on Monday.
"We'll wait and see," Arians said. "I think that's just the early prognosis of the exam we have right now. He'll go through the MRI and the rest of the tests and we'll just keep our fingers crossed."
If Mathieu is lost for the season, it would be a big blow to the Cardinals, as he has been the kind of playmaker they needed at safety after cutting ties with 2012 starters Adrian Wilson and
. He has 64 tackles, a sack, eight passes defensed and two interceptions and will likely earn consideration for Rookie of the Year honors.
"To me, he's a lock," Arians said. "No one else has played as much as he's played, and all the positions he's played. I just wish (the Rams had) kicked that damn ball to
Mathieu has played in 12 games and took over for
as the starter in Week 4 in Tampa Bay after Johnson lost his fingertip on a freak play against New Orleans in Week 3. Johnson replaced him on Sunday and said he's ready to resume his old role.
"That's what this league -- it's a league of great players, and when somebody goes down, an opportunity presents itself," Johnson said. "I'm ready. There's not going to be a letdown on the defensive side.
"I was able to accept my role when it grew smaller. Now it's going to grow, and I'll still have to do what I need to do."