Too many people are saying this. Sorry, I just don't see it that way. He narrowly lost AO 2012 against Djokovic - the current king of AO - and previous to that he made the USO final.
If he comes back good enough to win RG, why can't he make the final days of the HC majors and give himself the chance of winning them, like he did earlier this year?
It was an exaggeration and it is possible, but it will be very hard for him to win more hard slams. Nadal is clear fourth favourite for the AUS 2013 open and it will be very difficult for him to win that tournament given the long lay off and the lack of match practice. The US open doesn't suit his game, as well, and he won't play to the 2010 level again on hard. Nole and Murray are clearly much better than him on hardcourt and he really needs to avoid playing them back to back in order to have a realistic shot.
He turns 27 next year which is when a lot of great players have not won any more, or very few grand slams, and it is very hard to tell how long it will take to regain a high level of play after such a long injury. Del Potro took a whole year and while this injury is not as serious his recovery time will be much longer due to age.
Overall I would give him about 10% chance to win the AUS although it is hard to price accurately until we can see his level after the layoff. That chance could be higher for future hard slams if Nadal comes back at a high level, but he will always be a big underdog.