Very, very difficult.
What I see, even though Federer is much older than Rafa, still he is winning tournaments. I don't know when was the last time Nadal won a tournament. Roger won a tournament final just the other night. Nadal is good, infact great player, but please wake me up when Nadal wins even 15 grandslams. Let's see the longevity of his success. Talking about US OPen, I believe Roger might be back at no. 2 or maybe no. 1 you never know. Djokovic might just slid away if he is caught up with injuries.
Don't think he will win more than 12-13.