Rafa has loads of points to defend- and then after Wimbledon none to defend so any fall could be temporary.
If you look at it as a whole season Andy has just 900 points left to defend in 1000 ATP events whereas Rafa has over 2000. That excludes just Miami the only one played in 2015. Nadal had five bad events if that is only making the Quarters or less whereas Andy had seven.
If Andy continues with current intent to play well in the other ATP events not just slams he should make a large nett gain on Rafa.
My own view is that Andy will gain far more than Rafa in 2015. They both had time out, and time recovering but playing, in 2014 but the opportunity is there for him this year to keep in top four or higher quite easily. If Rafa is lower ranked he will have to go through the prospect of harder games from quarters and if he does not make it through them it gets harder and harder for him. Ask Andy how difficult it is to start an event ranked 5-8 as he did often in 2014. I think he has learnt that lesson?