There are 2 differences between Nadal and other players who have come back from long term lay offs:
1) Nadal is an 11 time Grand Slam Champion at the age of 26
2) Despite being absent for 8 months Nadal will still be ranked number 5 when he comes back so will retain favourable draws through his seeding: He will likely be seeded top 4 at Wimbledon
Form is temporary: Class is permanent and by any measure Nadal would have to be ranked in the top 5 players of the past 30 years. If he retains his determination and desire his comeback will be successful.