It's probably the only thing that could motivate him now. I would fully expect him to get back in shape, get his head in the right place, and focus intently on fighting AJ, for one last potential hurrah. It would be an awesome fight, and probably quite similar to the one he had with Klitschko, albeit with a slightly different result, I believe.
Nobody should make the mistake of underestimating Fury - for all that he comes across as a clown sometimes, and despite the fact that he's no elite athlete or power puncher, he's a natural boxer, with incredible fighting intelligence. The difference this time, I believe, would be that Joshua is less cautious than the latter-day Klitschko, and is much more likely to throw a lot of heavy shots, especially knowing that Fury isn't as likely to counter with anything like the power of Vlad, and he would finish him at some point, probably several rounds into the bout.