It is very difficult to be a great player for a long time at the highest level of hockey. Aging, the force of competition, and overall evolution of the game usually end up gradually pulling them back to the pack, a process that can be accelerated by injuries, coaching strategies or team environments. We have seen some of the league's best players go from special to ordinary because of injuries, Eric Lindros being one of the most extreme examples. When that happens it is obvious to everyone that the player is a shadow of their former selves.
Sometimes the descent is a little more difficult to pick out. There have been players who went from special to merely very good because of injury. Wayne Gretzky's injury in the 1991 Canada Cup has often been used to explain his late career scoring drop, for example. For Gretzky, a "drop" still meant being among the league leaders in assists every year, but in his later career the Great One was not nearly the same goalscorer or even-strength offensive player that he was in his prime.
I think one overlooked example of this latter type of injury was Dominik Hasek's groin injury in 2000. The numbers suggest the injury was what ended the Dominator's peak. Many people consider the 2000, 2001, and 2002 seasons to be part of Hasek's prime, but I don't believe that is correct.
To demonstrate this point, we need to look at the situational breakdowns. There were a couple of things going on in the early '00s that were inflating Hasek's numbers: A league that was gradually becoming more low-scoring, and a Buffalo Sabres team with improved team discipline that took fewer penalties. Let's look at Hasek's even-strength save percentages:
We don't have official even-strength save percentages from before 1998-99, but based on my estimates from Hasek's overall save percentage and the number of power plays the Sabres faced I'm quite sure they were much closer to .946 than .925. In 1997-98, for example, NHL.com has the goals against by situation, although the shots faced totals aren't correct. However, if we assume that Hasek faced the same number of even strength shots against in 1997-98 as he would in 1998-99, his even strength save percentage would have been .938. That assumption is unrealistic, given that Hasek played more minutes and faced more shots per game in 1997-98 than he did in 1998-99. Hasek was probably performing at .940+ in terms of EV SV% in '97-98, and he was likely in or near that range for the entire period from 1993-94 to 1998-99.
If old age was the reason for the drop from '90s Hasek to '00s Hasek, then we would have expected a more gradual decline. There was essentially a sudden drop from a high peak to a lower plateau, which fits the pattern of a player reduced by an injury.
Hasek's even-strength numbers recovered post-lockout to the .930 range. Perhaps that year off allowed him to heal some of the old battle scars and revert to the Dominator of old, at least for half a season. After that, another injury at the Olympics and the effects of old age caught up with him, even though Hasek was still an effective goalie pretty much right up until the end.
I have heard it argued that comparing Hasek's Buffalo numbers with his numbers in Detroit show that it is more difficult to dominate as a goaltender on a strong team, but I think that is wrong. Hasek's numbers dropped in Detroit partly because the Red Wings took a lot more penalties than the Sabres, and partly because he was not the same goalie that he was at his peak.
It is not surprising that Hasek ran into injury trouble, given that his style had to have been one of the most physically demanding styles of any goalie to ever play. He usually played a straight butterfly on the first shot, which applies the usual stress on the groin and hips, and then he also added his special brand of post-save moves and close-save tactics that required extreme agility and flexibility. Unfortunately, injuries often follow athletes who play a style nobody else is able to play (think of guys like Orr, Lindros, Forsberg, etc.), and the same thing was true for the Dominator.