The conditions will be way different from their October encounter where according to some, red clay was muted by weather and heavy damp conditions which negated some of the Nadal topspin. The ball will be bouncing and spinning on a nice hot day. Now on this surface Nadal has more time to set up DTL forehands and backhands , which he needs to break up the pattern of rallies where Novak thrives on.
Can Novak win, of course. How? Naturally high first serve percentage. Second of all, on HC he's so good that he can just stay neutral in most rallies, but not against Nadal on clay, so he has to take risks and get more aggressive to keep Nadal off balance, ie mix in Soderling. Next, shot selection, a middle ground from just defaulting to drop shots galore to really mixing it up. Novak can't get passive or he's sunk. And contrary to popular opinion, Rafa can't just outlast Novak on red clay, I gave the endurance edge to Novak. With Novak he usually lacks the self discipline to stick to the plan, because Nadal is unrelenting in his topspin onslaught, but Rafa can hit short and Novak has to take advantage of that. IF he does this for probably five sets with no major dip he can win.
However that's alot to ask.