Mexico striker Hernandez scored the winner in United's 3-2 victory over Stoke on Saturday, but believes De Gea was the real hero, as he kept the champions in the game.
Hernandez told the club's official website: "He was wonderful.
"Like I always say, we are a team and there are no heroes, but in some games there are players who make the difference and David was the one on Saturday.
"He's a wonderful goalkeeper, for me he's probably the best one in the world.
"People don't like to say that but, for me, he probably is. David and Edwin van der Sar are two of the best goalkeepers I have trained with - wonderful. Same level, same style and David is improving day by day."
United boss David Moyes picked out De Gea for praise in the dressing room and Hernandez added: "That's what the gaffer said to us and we agreed that it was because of him we got the three points.
"He made two or three saves after the goals and he was just fantastic."