An MRI exam Monday morning confirmed the injuries. Mathieu will have to wait to allow the LCL to begin healing before he has surgery to repair both ligaments, but Arians said the procedure could take place as soon as eight days from now. Recovery could take about nine months, sources said.
"This will be a long one. But I won't attempt to say how long because it's [different with] individual guys now," Arians said Monday, adding that its doubtful Mathieu will be ready by the start of training camp.
The hit occurred with about nine minutes left in the third quarter on a free kick return after the Cardinals recorded a safety. Mathieu got up and began jogging off the field when his left knee buckled and he fell to the turf at University of Phoenix Stadium. He was carted to the locker room and did not return.
Cardinals cornerback Patrick Peterson, a former LSU teammate of Mathieu's and one of his closest friends, showed his support via Twitter on Sunday night:
Man I just want to wish my boy @Mathieu_Era for a speedy recovery. Hurt me to my heart to see a King go down like that. We gone hold it down
— Patrick Peterson (@RealPeterson21) December 9, 2013
Mathieu has become one of the league's top rookie defenders after falling to the third round in April's draft and was a candidate for defensive rookie of the year. During his postgame news conference Sunday, Arians said he thought Mathieu should win the award.
He repeated those sentiments Monday.
"I still think he should get rookie of the year defensively. The job he did in the three positions that he's played was outstanding. I don't know if it'll hurt him to where somebody else will get it. There's some other deserving guys out there also. He meant a lot to us because of the flexibility in his positions. It'll take two guys to take his place," Arians said.
Mathieu has 63 tackles -- second-best on the team -- along with two interceptions and a forced fumble. He was named NFC Rookie of the Month in October.
Mathieu, who became a full-time starter in Week 4 against Tampa Bay, will be replaced in the lineup by safety Rashad Johnson 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.
After the initial shock of his injury wore off, Mathieu reached out to fans on Twitter.
I appreciate everyone prayers. We go through things in life to test our faith & strength..
— Tyrann Mathieu (@Mathieu_Era) December 9, 2013
ESPN NFL Insider Adam Schefter contributed to this report.