NFL player Chris Borland retires after one season at age 24 due to head injury concerns
49ers All-Decade: Which stars make the cut? – Daily Democrat
Report: 49ers expect Patrick Willis to retire
Patrick Willis Says Change 'Must' Happen After 49ers Loss
Bleacher Report - San Francisco 49ers make it official: Patrick Willis will retire. Details
Rough offseason continues for San Francisco 49ers
NFL Star Patrick Willis Now Working in Silicon Valley
Report: Former Ole Miss great Patrick Willis to retire
49ers LB Patrick Willis retiring after injury-shortened season – New York Daily News
49ers Rumors Roundup: Patrick Willis, Justin Smith To Retire
Report: 49ers' Patrick Willis planning to retire
Patrick Willis Retired in His Prime, But Where is He Now? - FanBuzz
Salary cap implications of Justin Smith, Patrick Willis retirement - Niners Nation
Dick Butkus salutes retired 49ers linebacker Patrick Willis – The Mercury News
Patrick Willis Retirement Stuns 49ers