Hoek says he wanted to shake up the keepers' routine when arriving with Louis van Gaal - something the likes of De Gea took some time to accept.
He told Sky Sports: "If you've been used to doing something in a certain way throughout your career, you're the goalkeeper of Manchester United and I'm suddenly going to do something else with you, then you're automatically questioning that.
"But I'm not afraid of things I think they make a goalkeeper better. I don't see a goalkeeper as a player apart from the rest. I look at the big picture.
"It's about the relationship between a goalkeeper and the team."
Hoek also said: "From individual training sessions they didn't always get better. In fact, they got worse. Imagine what individual goalkeeper training looks like you have one player who fires shots. How often does such a situation happen in a match? If the assistant coach fired shots at me, I knew everything after one training session, finding out about his shooting style. There was no surprise element, nothing. Not only that: when it comes to competitions, you defend together and attack together. So you have to work together to read the match situations."