In tackling questions like these, the tried and tested answers usually boil down to semantics. Examples of this being: "Nadal can win the US Open but no-one knows if he will" or "Nadal will likely perform better over five sets than he has, of recent, over three sets."
However, the simple answer to this question from current evidence is no.
It is hard to be definitive about a player's form—especially a player like Rafael Nadal—without first-hand information from the player himself. Nevertheless, performances speak for themselves and they can give us a gauge of where the player is mentally and physically.
2009 was a painful year for Rafael Nadal—personal problems off the court distracted him and he was plagued by injury. He ended that year winning just the one slam.
The most poignant thing about 2009 was that it came right after Nadal had proved, the year before, that he was more than a one-slam-a-year player. In winning just the one Grand Slam title that year, Nadal conceded a huge amount of ground to Roger Federer in the race for the most Grand Slam titles. It visibly affected him.
2010 afforded him much needed respite, as he won three of the four Grand Slam titles and ended the year as world No. 1. 2011 was supposed to follow that trajectory but it hasn't. Nadal won the French Open unconvincingly and lost with a whimper in the final of Wimbledon. This year he has lost all of the five finals he has played against Novak Djokovic, the current and deserving world No. 1.
It's a massive turnaround. It's not helped by the fact that we are heading into the US Open which is played on hard courts—Nadal's worst surface. With Novak Djokovic in imperious form, and Roger Federer and Andy Murray being better hard court players than Nadal, Nadal's chances of defending his title are remarkably low.
More damaging to Nadal though, is that without a win at the US Open this year, he would reenter the dark night of the soul that was 2009. He would've won just the one slam (which is totally unacceptable for a player of his quality, with 2010 in mind) and he would again concede ground to Roger Federer and his record haul of 16 Grand Slam titles.
Nadal is only human, and there is only so much that can be stomached. If 2009 broke Nadal as a person, 2011 is breaking him as a player.
Speaking on the new challenge he faces as a player on tour, Nadal said:
I don't have a personal rivalry against Novak. I have a rivalry against everybody. I try my best every day when I go on court. This year, Novak is doing fantastic things, fabulous what he's doing, it's really amazing, and probably impossible to repeat for the rest of us.
I want to congratulate him, but for me the challenge is not different. I have a goal right now, and that's improve my tennis to be enough good to compete against him for next year.
For this year, I don't have enough time to practice, but hopefully for next year, yes. I have to change and improve a few things, and that's what I going to try.
It is unlikely that Nadal will win the US Open but, as 2010 showed, it is likely that Nadal isn't done winning.