The striker told Champions magazine: "Rafa has taught me to demand a lot of myself daily, not to relax. When you are in a team where things are going mostly well, players tend to relax.
"After you score 20, Benitez is on at you during the next training session and stays on top of you all day, all week. He tells you to go off on your own and do new exercises.
"There are days when you think, 'My God, this guy does not let you breathe'. At times like that you don't realise that all he wants is for you to improve.
"I want to progress in life, not relax, not be complacent. And you need someone close to you telling you to keep at it. It is impossible to do it otherwise.
"We don't always fancy having someone like that so close. But in the long term, I'm sure everybody will thank him for it."