Bills cornerback Vontae Davis, who retired during halftime of the team's loss to the Chargers, released a statement via the NFL. He apologized to his teammates and coaches, but says he doesn't have what it takes and shouldn't be on the field anymore.
If you don't have it anymore and don't think you should be out there you could still wait until the game is over. Fake an injury or something and wait until Monday to retire. This has to be the only halftime retirement in NFL history.