The Spanish youngster has had plenty of criticism since his move to Old Trafford in 2011, though his performance in the 1-1 draw with Real Madrid has been widely praised.
"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. But I think he is doing brilliant and in a couple of years he will be the number one in the world."
De Gea has made 27 appearances for Sir Alex Ferguson's side this season and seems to have cemented his place as first-choice goalkeeper.