Although Fed is considered as a very proud man and Nadal as a humble bull, I think it's obvious that if Nadal drops out of top 10 by losing at RG early, or start to get whipped by Fognini, Raonic, Verdasco, Berrer, Coric, Deliciano, Klizan, Kyrgios, Brown, Almagro, Dolgo, Darcis and Rosol on regular basis*, then it is safe to say he will be finished before Fed.
Whether he will go with injury or will he retire while he is still healthy, it's just up to him, but I must admit it would be an ugly picture and it will cast a shadow over his achievments.
Nadal looks more like old women than a young man in his late 20s. What caused this, I really don't know and I don't want to speculate about, but it is sad to watch someone deteriorate so much. I just hope his physicality will stay with him, because he can be persistant and he could turn things around at some point.
In 2 weeks, we will know some things better, possible even sooner.
*all those players beaten Nadal recently, but it's their rank what matters the most