There are certain situations where you can say a fighter is not 'prime' such as the Ali who came back after years off suspended or the Ray Leonard who returned to fight Hagler. However, I think people often tend to use it as a way to excuse a loss by a previously unbeaten fighter who looked unstoppable, such as in the case of Tyson or Don Curry. I can agree that the Tyson who came out of jail was not prime, but the guy who fought Douglas, underprepared or not, was still a prime fighter.