Tyrann Mathieu has insisted that he has not lived up to expectations in his first season in the NFL.

The Arizona Cardinals safety is currently the favourite to be named defensive rookie of the year, having made 67 tackles, a sack, a forced fumble and two interceptions in his first 12 NFL games.

The former Heisman Trophy runner-up told the team's official website: "Actually, I don't think I have lived up to my expectations. There are a lot of plays out there in previous games that I feel like I left on the field.

"There is definitely growth that I can make and try to make those plays up these last four games.

"I'm getting comfortable but I've been comfortable with the defense and stepping into the starting role, having to be a leader out there. It makes me have more responsibility, but that's why you play the game."

Mathieu missed the 2012 college season for breaching team rules, spending a brief period at a drug rehabilitation facility.