On September 5, 2009, he became one of two undrafted rookies, the other being Julius Williams, to make the Jaguars opening day roster.
Allen was named starting weakside linebacker on October 11, 2009 against the Seattle Seahawks. During the 2009 season, Allen played in all 16 games, starting five.[1]
Allen suffered a stroke on December 15, 2013 during a game against the Buffalo Bills. Allen was placed on injured reserve on December 19, 2013, ending his season. Doctors subsequently advised him to avoid playing again.[2]
Allen was released by the Jaguars on April 17, 2014.[3][4]
On April 22, 2014, he announced his retirement from the NFL at the age of 27.[5]