Great trainers? Who is considered a great trainer? Angelo Dundee? Cus D'Amato? Freddie Roach?
OK. Now that we have a few names let's try to think how did these people affect on their boxers.
Angelo Dundee is probably world's greatest boxing manager. He has worked with the likes of Muhammad Ali, George Foreman and Sugar Ray Leonard. Dundee worked as a "water boy" for great trainers in the gym which his brother opened. There he learnt many strategies and tactics for boxing which he later deployed on Muhammad Ali.
Cus D'Amato on the other hand wanted to become a professional boxer, but his dream was destroyed when in a street fight he lost the ability to see with his left eye. Later a future world champion was stolen from him by the mafia, but he never gave it up. He kept believing and he made such champions as Floyd Patterson and later Mike Tyson. His work with Mike was what made him one of the greatest trainers of all time. He was able to communicate in a very unique way with his boxers and explained to them the true nature of boxing.
Freddie Roach is like the Jose Mourinio of boxing. He is young but yet he is very successful. He works with Manny Pacquiao, he worked with Mike Tyson, Oscar De La Hoya, you name it.
So great trainers do affect great boxers after all. I'd say that great trainers and great boxers go together as one and they can't live separated.