Manchester United's £18.9m Spaniard - forced to share duties with Anders Lindegaard during his first season at the club - has been accused of weakness in dealing with high balls since he arrived in the summer of 2011.
But De Gea has made the No 1 shirt his own this term, and an eye-catching performance at Real Madrid in midweek brought praise from his critics and Sir Alex Ferguson alike.
Now Hernandez has given the boldest assessment yet of his 22-year-old team-mate's potential.
He said: "David is doing brilliant and in a couple of years he will be the No 1 in the world.
"A lot of people who know about football know that David is a top-class goalkeeper. 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.
"David is very focused and doesn't care about what people say about him. He has a lot of confidence. He doesn't need to listen to what people say about him."
United boss Ferguson, meanwhile, has dropped the clearest hint yet De Gea will remain at Old Trafford next season despite claims he will move back to Spain next summer, with Stoke's Asmir Begovic joining the Red Devils.
"What people say didn't worry us before, so why should we worry afterwards," said Ferguson.
"There will still be occasions when he finds it difficult in terms of the physical part. But he is getting stronger and next year he will be better."
De Gea's place is at risk from Lindegaard for Monday's FA Cup fifth round tie with Reading given Ferguson has pledged to make changes.
The Dane has only made one appearance since a dismal performance the last time the two sides met at the Madejski Stadium in December when he was condemned for his contribution to goals conceded in a 4-3 win.
Victory would keep United on track to emulate their 1999 Treble-winning campaign. So, whilst it would be easy to let minds wander towards that return leg with Real on March 5, Hernandez is keen to focus on the present.
"Wednesday was a very good result for us but now we need to think about the FA Cup," he said.
"If we want to win the three competitions, you have to think game by game and try to get the points in the league and get further in the cup."