After John Abraham sacked quarterback Kellen Clemens for a safety, the ensuing free kick was taken by Mathieu and when he was tackled, his legs ended up in the air and got tangled up with the players who took him down. He had to be carted off and did not return. The way his teammates huddled around him, it didn't look good.
After the game, head coach Bruce Arians said that the injury "appears to be" the ACL, which could end the rookie's season. That news provoked an audible groan from the media members.
Mathieu himself tweeted this before the game was over.
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
A teammate told reporter Kent Somers that Mathieu was "bawling his eyes out" after he was told what the injury could be.
He will have an MRI done on Monday to determine the extent of the injury and whether or not he will be able to return this season.
Including the game against the Rams, Mathieu has collected 68 tackles, a sack, a forced fumble, two interceptions and nine passes defended. He has been in the discussion for both a Pro bowl berth and Defensive Rookie of the Year.