Duges is managing a chronic knee issue presumably pretty similar to what quite a few 10ish year NRL players have to, mostly just wear and tear. Managed via reduced loading more than anything, which is the opposite to what Moylan needs, he needs to get his strength and fitness in his legs right up which isn't possible when he has to play on the weekends. Missing all of pre-season, whilst related to the issue, also maximise the chances of the issue cropping back up during season. Hammy recursion rate is one of the worst so the more fitter stronger muscles he has around it supporting it the better his chances of not aggravating it.
Morris says he will manage Moylan at FB by using Dugan to drop back there to do some defending, which I personally don't love, and thinks he can get Moylan fine if he can get him through a full pre-season (which would need to be very very carefully managed).
Or at least, that's what he says publicly. I've been pretty clear I'd be happy to see Moylan call it a day after this season and I think most, if not all, on here are on board with that idea.