A neck injury will prevent the 28-year-old McNeill from playing again, Keels said.
McNeill played six seasons in San Diego, where he signed a six-year, $48.895 million deal before he was released last March.
McNeill injured his neck during a loss to the Oakland Raiders on Nov. 10 last season. He was placed on season-ending injured reserve later that month.
A two-time Pro Bowler, McNeill was a second-round draft pick of the Chargers in 2006 out of Auburn. He will finish his career with 82 starts and two fumble recoveries.