IMO, this is just further proof that some guys are, in fact, injury prone, and not merely the victim of bad luck. If it happens 4, 6, 8 times, it's not a fluke thing; there's some inherent flaw in the person's genetic makeup, and that's not something you can escape with any amount of conditioning/treatment/etc. (Though it can certainly be called "bad luck" to end up with that genetic makeup in the first place, but that's not the point.) Rob Williams is injury-prone, period. He'll probably never come close to playing a full season.
Durability is certainly an athletic ability, you're right, but I also think there's a wrinkle with guys that become injury-prone. Hayward springs to mind, he had a pretty durable run in seven seasons with Utah, but he's cracked 55 games just once since, and that was his minutes-restricted 'real' debut with us. Would you say Hayward's a victim of bad luck?
I'm not sure what I'd label Hayward. Mainly, my cynical side looks at Hayward (and Kemba) and thinks, "They were ironmen before Boston, then started breaking down as soon as they joined the Celtics. How lovely."
I guess I would say his initial injury with Boston was bad luck, because he got hurt on what was not a typical play, but I haven't paid enough attention to his post-Boston injuries to know how to categorize those. With Rob, though, he's not getting hurt on fluke plays; he's getting injured while going through normal, everyday basketball movements.