Shkodran Mustafi’s header from a corner was palmed up into the air by the Spaniard.
The ball then looped behind him where Ander Herrera cleared his lines.
But VAR confirmed that the ball had already crossed the line.
Fans took to Twitter to express a mix of emotions after De Gea’s blunder.
One fan said: “Did De Gea take that in? Looked like header was going over to me.”
Another said: “De gea has been finished for at least a year GET HIM OUTTTTTT.”
Another joked: “Thought De Gea was the best keeper in the league.”
“People still think De Gea is better than Ederson in 2018. lol,” said a fourth.
The goal seemed to follow the recent pattern of goalkeeping blunders.
Jordan Pickford cost Everton the Merseyside derby on Sunday when he gifted Divock Origi a 96th minute winner.
The England No 1 has been linked with a move to Old Trafford should De Gea be allowed to leave.
“That was poor from De Gea, should’ve been easy save,” another said.
Another fan tweeted: “De Gea is allergic to cleansheets nowadays #MUNARS.”
One United fan defender his goalkeeper: “De Gea is allowed to make mistakes, he’s earned that right.”
Another said: “Someone tell me why De Gea is still spoke about as one of the worlds best this season.”
One fan made a huge claim about the goalkeeper. “Unpopular opinion: David De Gea is the reason for United’s poor defence this season. He needs to be replaced sooner than Young or Smalling. Bang average keeper.”