De Gea has come in for huge amounts of criticism at times following his arrival from Atletico Madrid in 2011, most of it centred around the difficulties he has had dealing with crosses and the physicality of the Premier League.
There was still some evidence of that weakness on Wednesday night, when De Gea produced what is widely acknowledged to be his best appearance for United, making a string of fine saves in the Champions League draw with Real Madrid.
As a Spanish speaker, Hernandez has developed a close relationship with De Gea during their time at Old Trafford.
And as someone who was forced to bulk up in order to deal with the buffeting he gets in England, the Mexico forward is well-placed to assess De Gea's ability to meet the unique requirements of life in his new home.
It leaves 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."
Sir Alex Ferguson has noted similar traits.
And the United boss 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 Anders 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."