So we all agree that injuries is part of the sport then this thread is pointless. Every players has their own problem, some are even worse situation than Nadal. Who's blaming on Nadal? It's his choice to sacrifice his body for his success. The unlucky one are the one that has a freak accident(e.g. Blake broke his neck). Nadal is not unlucky.
^^^ This.
As many have already said, almost everyone was predicting Nadal's style would lead to injury, but he and his team took a calculated risk. He's got some big prizes, but the cost may be retiring younger than some of his peers, but hopefully he will know to stop before he carries an injury that will have a considerable impact after he's retired.
Most players experience injury or ill health during their careers, and many of them will have played at least some tournaments while still recovering, giving their opponents an advantage.
As far as I'm aware, there was no underlying weakness in Nadal's knees. Just normal human knees that can only take so much. If you are going to play the "what if ..." game, you would do far better to wonder how Murray would be if he didn't have a bipartite patella. Unlike Nadal, he seems to have actually modified how he plays to stop that from becoming a much more serious problem, and is quite probably is a factor in him never quite getting the movement right on clay.
There is nothing special about Nadal's injuries compared with other players, except they are always used as an excuse for defeat.
You could extend this thinking to all players that retired when bad backs and aching joints became more trouble than it was worth. What if David Ferrer was 6"1' like Nadal? What if Nadal were right handed? What if Djokovic didn't need to wear contact lenses?
The reality is that all players have to make the most of the body and natural talents they are given. There is a certain amount of luck in how often, when and how badly you get sick and injured, but part of being a professional is managing that, and part of being on tour is accepting that you won't be able to play every tournament in peak physical health.