The NHLPA has to persuade the arbitrator that the differences between the Kovalchuk and Hossa deal are insignificant, and that, if Hossa is allowed, Kovalchuk must be. They will need to to convince the arbitrator that the odds of Hossa making it to 42 are no different than Kovalchuk making it to 44, i.e. neither is reasonably likely.
But the NHLPA also has to be mindful of the possibility that the arbitrator may decide (or already believe) that the Hossa contract ought to have been rejected in the first place. Saying Kovalchuk is just like Hossa only works if the arbitrator believes the Hossa deal is solid. If the arbitrator thinks it’s not…well, there’s not much the NHLPA can do. I guess the game would be over at that point.