To state the obvious, we can't say anything until we have seen him play. Assuming he's back to, say, even 75% level of what he was in 2012 before he pulled the plug on the year, he'll win RG, and probably be a favourite over Murray and Federer in any grand slam. He might be slightly more susceptible to upsets by Tsonga/Berdych/Del-Potro but like I said, 75% Rafa is still top 4 everywhere and still number 1 on clay, and has the edge on Federer and Murray in all grand slams.
Rankings aside, I can easily see him being the second best player this year.