nadal's playstyle -> slam wins and nadal's playstyle -> dodgy knees so slam wins and dodgy knees are a bundle. You can't have one without the other.
No dodgy knees means nadal wins a lot less slams. nadal's tactic is to tire his opponents and force them into errors. It's almost like he makes the opponent lose the match instead of him winning it.
Lucky for Roger talent doesn't equate physical problems. Too bad nadal is severely lacking on that department.