Physically unimposing at first glance – 5-foot-9 and 175 pounds – and with the quiet unassuming nature off the field of a gentle kid, Mathieu was just another guy in a uniform as the 2010 season crept up.
Then the Tigers kicked off the year against North Carolina and Mathieu's role changed once and for all.
On a night when LSU needed every defensive playmaker it could scrounge up as the Tar Heels staged a second-half comeback, Mathieu blossomed in fast-forward speed. He logged nine tackles, forced a fumble, sacked UNC's T.J. Yates once and was in on another tackle for loss.
More than just statistics, though, Mathieu did something nobody expected, at least not that quickly: He looked like he belonged.
As Mathieu's evolution played out, Tigers All-American cornerback Patrick Peterson mentored the freshman and became an important touchstone and sounding board. The two grew so close, that when Peterson left for the NFL Draft, he offered Mathieu his No. 7 jersey.
Now, with the LSU secondary stockpiled with talent despite losing Peterson, Mathieu is a familiar face and a quiet leader – one who doesn't have a set position, but instead is poised to be the extra defensive back in a defense that figures to feature a lot of 4-2-5 formations against a series of opposing offenses that will look to pepper the Tigers through the air.
Officially, Mathieu's primary position will be as the nickel back – the fifth DB who has some freedom to roam and hunt down the ball. That means Mathieu might technically not be listed as a starter. Just don't expect any bruised feelings over that.
What's abundantly clear is that LSU's defense is better with Mathieu on the field. Could him playing nickel back make the Tigers even more potent or be a more natural fit?
LSU coach Les Miles has repeatedly praised Mathieu's instincts and anticipation, which blended together, seems to put the former St. Augustine star around the ball on almost every snap.
When Mathieu emerged as a budding star last season, his uncle and guardian Tyrone Mathieu recalled stories of when he first started playing as a youth and was so consistently in the backfield almost simultaneously with the snap that opposing coaches pleaded that he be moved to an older age group.
The opposition has caught up a little bit, but Mathieu still thrives with a knack for being in the right place at the right time – or wrong time if you're in a different colored uniform.
"Nickel back is a mental game for me," Mathieu said. "It gives me a chance to use my instincts to make plays, and that's one of my biggest strengths. At cornerback, you focus on your man and trying to control the line of scrimmage. At nickel, you're in a roaming position and get a chance to put yourself in the best position, and that keeps me closer to the ball."
And when Mathieu is on the field – and especially when he's around the ball – LSU's defense is at its best.
That explains why Miles and defensive coordinator John Chavis have spent the offseason tweaking and molding the Tigers' defense to make sure three players with the skills of Morris Claiborne, Tharold Simon and Mathieu are on the field together and not jockeying for playing time.
"It's about getting as much speed as we can on the field," Chavis said. "That's what we need to do and we're never going to sacrifice speed for size. We're going to put as much speed on the field as we can."
Added Claiborne, a pre-season All-SEC pick who led LSU with five interceptions last season, "If we're doing what we're supposed to, it's going to be pretty tough to the throw the ball against our defense with much success."
Another thing that has made it tough to keep Mathieu off the field has been his accelerated cerebral development in Chavis' scheme.
"Nobody has to tell me what to do any more," Mathieu said. "I know where to be, and it's on my shoulders to take control of the defense and work well with all the guys around me. I also have to work well with the cornerbacks and the linebackers, and I need to know what the safeties are doing behind me."
Not that there won't be ample chances to freelance.
Like Peterson and Chad Jones before him, Mathieu will have a chance to capture lightning in a bottle as a punt returner. Miles said Wednesday during his radio show that Mathieu "could be every bit as explosive as our best" at punt returner.
Both his immediate predecessors scored touchdowns in that role, a notion that causes Mathieu's eyes to light up.
"I can't wait to get the ball in my hands in that situation and see what I can do," Mathieu said.