If the player is very old already, the "career ending" injury doesn't have to be a particularly major one.
Let's say your favorite player is 37 years old and scores four touchdowns in an exhilarating Super Bowl victory, including the game-winning TD as time expires. He is hit by four opposing defenders at the 1-yard line, who break his left ankle but still can't stop him from reaching the ball over the goal line.
He wins Super Bowl MVP. Doctors tell him his broken ankle will take "only" four months to heal and then he can play again, but he says, "Nah, I'm 37, I just won a Super Bowl, I got MVP, I'm retiring."