Hernandez is no doubt De Gea has the capacity to become the very best, very quickly.
"A lot of people who know about football know that David is a top-class goalkeeper," Hernandez told MUTV.
"That is why Manchester United bought him.
"They saw how good he can be. If he wasn't at the right level to play at this club he wouldn't be here and they are trying to look after him.
"To me, he is a complete goalkeeper. You see the great saves he made on Wednesday. He can also play with his feet, which is very useful to see in a goalkeeper.
"Other people are speaking about when he has to go out and clear the corners and free-kicks. They say that is the weak part of his game.
"But I think he is doing brilliant and in a couple of years he will be the number one in the world."
Although De Gea can apparently understand more English than he tends to admit outside the confines of United, he has not allowed the criticism to affect him.
"David is very focussed and doesn't care about what people say about him," said Hernandez.
"He has a lot of confidence. He doesn't need to listen to what people say about him.
"He should just play and enjoys it because he knows he can be number one in the world."