Is fear of injury pushing NFL players like Borland out?
Has there been another NFL player that was very talented that retired at an earlier age than Andrew Luck? - Quora
San Francisco 49ers Chris Borland retires over repeated head trauma
Chris Borland's NFL Retirement Cost Him Millions, But Could Add 2 Decades To His Life, Statistics Show
NFL players react to Chris Borland's surprise retirement - Sports Illustrated
Borland Retirement: Is 24 Too Late to Stop Football Brain Damage?
John Urschel retires from the NFL at age 26
Fearing brain injury, 49ers' Chris Borland retires after one year
Chris Borland's Retirement an Uncommon Choice—but One NFL Players Understand, News, Scores, Highlights, Stats, and Rumors
Borland Retirement: Is 24 Too Late to Stop Football Brain Damage?