De Gea is in contract dispute with United, but Hart insists they cannot afford to lose him.
He told Sky Sports: “His standards are ridiculously high.
"The way Manchester United have played over the last few years, it's not like he is having to make the odd save here and there, he literally has to make two or three top saves a game, plus three or four normal saves, for them to have anywhere near a chance of winning.
"The constant attack that he's under, unless he's in tip-top form, those stats will be as they are. A few things might have happened that he wouldn't have liked, but he's standards are so high.
“He's right up there, he continues to make save after save.
"We all know what happened when he first came in, I was playing against him, he was a young guy, 20 years old, and the mental strength and composure he showed...plus the faith United showed...has been repaid by winning player-of-the-season year after year."