In the NFL, great quarterbacks don't retire, they simply move to another team.
It happened to Johnny Unitas, Joe Namath, Joe Montana, Brett Favre and Peyton Manning. All were driving forces behind tremendous periods of success for their franchises and all ended their careers playing for someone else.
This year, it happened to Tom Brady and Philip Rivers, too. When Brady signed as a free agent with Tampa Bay despite winning six Super Bowls — including three of last six — in New England, it was a signal that no quarterback is immune once an NFL team decides to move on and embrace its future at the game's one indispensable position.