AFAIK it's just a rumour. He would have less money from sponsors, that's it. In a sport you don't actually need fans to succeed, like it's the case in other fields, like music for example. Even then, musicians don't owe fans a thing really. Fans buy records, go to concerts, because they enjoy it, for no other reason. Anyway, someone who's great at a sport will always have some degree of fan support. And Nadal is fan friendly enough... he always signs autographs, what would you want from him?.