He really isn't. I positively hate it when people say that because it becomes very much a self-fulfilling prophecy. How many times have we seen players get ahead against Nadal on clay only to crumble mentally just before they reach the finishing line? A lot of it is to do with Nadal, but a lot of it is also to do with them being told for the past eight years than Nadal is invincible on this surface.
Thankfully though Djokovic isn't so feeble-minded:
has Rafa come back from a set down against Novak in a grand slam to win before? I'll just check
Edit: Wimbledon 2007 - where Novak retired after 2nd set, otherwise no
He hasn't and its not happening on Sunday either Nadal's only hope is to win 1st set if he does he has a good chance other wise you know what's happening for the 1st time