Andy will need to serve really well. That will be the key for Andy. At least 65-70% is required. He's moving well but of course Rafa is Rafa on clay.
At least Andy has good experience playing Nadal on clay and has taken a set in MC in 2011. On the other hand, Rafa leads Andy on clay 4-0.
Andy has nothing to lose and should relish the moment and feel very confident. If he can serve really well and gets a good bite on Nadal's 2nd serve and moves well, then anything is possible. At the very least, he can try and hang in with Nadal as long as he can. That may pay off. But I'd prefer an offensive Andy with a bit of mixture of defense if or when it's required.