Description: A brain injury has forced former Jaguars linebacker Russell Allen to retire from pro football. SOURCE: Sports Illustrated
Insight: An on-field stroke against Buffalo in Week 15 left Allen with a dead spot on his brain. The injury was originally believed to be a concussion but was actually much more serious. Allen's best season came in 2012 when he recorded a career-high 131 tackles in 16 games for Jacksonville. The San Diego State alum turns 28 next month.