He was a mystery in the eyes of the NFL, a troubled college player whose professional skills were as unknown as the afternoon weather. But any questions about how the 5-foot-9 safety would handle the rigors of the NFL were answered on one play. Mathieu tracked down St. Louis tight end Jared Cook late in the first quarter, punching the ball out of his grasp and forcing a fumble.
The perception on the former Heisman finalist began to change. In a hurry.
St. Louis visits University of Phoenix Stadium on Sunday, 13 weeks after that first meeting of the season. Mathieu is now the Cardinals' starting free safety and a prime contender for defensive rookie of the year.
"I'm more comfortable," he said. "I think the game has slowed down for me a whole lot. I think my first NFL game, I think I was trying to do everything fast, just moving around fast and not really seeing everything develop. I think last couple weeks I've been able to see formations and just kind of have an understanding of what the team's trying to do."
That maturation has been noticeable to defensive coordinator Todd Bowles. With every passing week, Bowles said, Mathieu is becoming a more experienced defender, but it's his ability to process new information and stack it on the foundation Mathieu has already built that's helped him become one of the league's top rookies.
"He's seeing some things he hasn't seen before," Bowles said. "Mentally, he's handling them very well. The game's not too big for him. You sometimes forget he's a rookie because he's out there every play, but he is still a rookie. So he's learning things every week but he's handling them well."
One of Mathieu's toughest adjustments to playing in the NFL was lining up as a safety. Until he came to the Cardinals, Mathieu played cornerback throughout his career.
His athleticism has helped ease the transition to playing deep in the secondary but Mathieu has also put in the time. Cardinals coach Bruce Arians sees Mathieu grow "every day" in "every aspect" as a cornerback, nickel and safety.
"He's playing so many different spots and it's fun watching his growth because he works so hard on the practice field and in the film room," Arians said. "He's one of the smartest guys we have."
He's also one of the most effective.
Mathieu is second on the team in tackles with 62, has two interceptions, one sack and one forced fumble. But there's still work to be done in his first season as a safety.
"I think I've adjusted well," Mathieu said. "I think [there's] still some technique things, and technical things, fundamental things that go with being a safety I still have to work on. But other than that I think it's fared out pretty well so far."