I know its been reported by one person with knowledge about injuries, that its just basically been bad luck. But from a common sense perspective, anyone who has played sport at any level knows that the fitter and more flexible you are, the less likely you'll suffer injury.
That's not to say being fit stops all injuries, of course it doesnt. But the number of simple injuries Manly players are incurring I suspect are related to fitness. If you are not fit, you tire quicker and when you are tired and try to push yourself, you are much more likely to be unable to avoid situations where you are likely to suffer injury. Further, it also comes down to skill training. The question of how you tackle, how you fall etc, all contribute to the potential for injury.
Take Menzies for example. He was rarely injured because he was always super fit, knew how to tackle effectively and was flexible in his movement. He knew how to avoid situations potentially causing injury.
My impression of this Manly side is that not only are they questionably coached (just a lay opinion that) but they are not sufficiently match fit and that is the fault of the trainers and coach. Hasler, who was a fitness fanatic, knew what was required of himself and his players. How often was Hasler out injured and yet he was a workaholic on the field. So was Toovey.
This Manly side looks to me badly prepared in many ways. They try hard, but they are just not up with the pace.