Unfortunately there is way more to the story than just a failed physical.
According to Robert Klemko of TheMMQB.com, Allen suffered a stroke on the field during the Jaguars week 15 game against the Bills at the end of the 2013 season. Not only was the stroke undiagnosed until after the game, but Allen continued to play after it happened on what he described as a very unassuming collision with Bills center Eric Wood.
Now Allen's NFL career is over as doctors have told him that he has a dead spot on his cerebellum, the part of the brain that essentially controls your body.
Allen was a fan favorite in Jacksonville after making the team as an underdog, undrafted free agent in 2009. After making an early impact on special teams, he found his way into the Jaguars starting lineup as an outside linebacker the last two seasons.
Allen's injury is yet another sobering reminder of the sacrifice that players make every time they step on the field and we can only hope that the rest of his life isn't significantly impacted by this injury.