Arenas played college football for the Alabama Crimson Tide, earned consensus All-American honors, and was a member of a BCS National Championship team.

He attended Robinson High School in Tampa, where he was a first-team All-Suncoast selection in football and a track star.

[5] Arenas attended the University of Alabama, where he played for coaches Mike Shula, Joe Kines, and Nick Saban from 2006 to 2009.

[6][7][8] In the 2006 season, Arenas recorded two punt return touchdowns, one against Florida International and one against Oklahoma State in the Independence Bowl.

[14] In the 2008 season, Arenas recorded one pick-six, which came against Arkansas,[15] and three punt return touchdowns, which came against Tulane, Mississippi State,[16] and Utah in the Sugar Bowl.

[25][26] The 2009 Crimson Tide completed an undefeated 14–0 season, beat the Florida Gators 32–13 to win the 2009 SEC Championship Game.

[37][38] The Chiefs previously traded tight end Tony Gonzalez to the Atlanta Falcons to acquire the pick used to select Arenas.

[59] His brother, Armando Murillo, was a Golden Helmet winner at Robinson High School in Tampa, and started at defensive back for the University of Nebraska in 2007 and 2008.

[60] On May 21, 2011, Arenas graced the cover of Sports Illustrated after an EF4 tornado tore through Tuscaloosa during the April 2011 Super Outbreak.

[61] Arenas is currently assisting the defensive backs and punt returners at his alma mater, The University of Alabama.