Far better? The Kings would have won a wopping 4.5 more games if Purcell played every game (based on the winning percentage). Only 6 more games if Moller played in all of them.
Right now the Kings are 17 games better with Mitchell based on an 82 game schedule.
0.733 vs. .523 is vastly different than the players you mentioned. Mitchell plays 20+ minutes per game in all situations, he has a mich bigger impact on the game than Purcell or Moller ever do.
I think most would agree that having a healthy Mitchell in the lineup makes the Kings a better team and the record shows that.
... The percentage difference just reflects a smaller sample size. Not saying Mitchell's a bad player and obviously the Kings would prefer him healthy, but let's not kid ourselves about him being the key to the team's success based on coincidental information.