nadal should have gotten heavily into cycling starting in 2009 and maximized match play and practice on clay courts effective immediately after that slam.
right now he would be sitting on 3 more slams and no knee trouble.
he was actually having knee trouble as he was winning the australian open in 2009. so that was to time to lay off the hard court activity as much as possible.
he did this to himself. he killed his own career while in his prime.