The Spaniard barely touched the ball in the first half but was called into action soon after the break to smartly stop a Wayne Rooney drive with his legs.
He later denied Gylfi Siggurdson – the last player to score past him at Old Trafford, all the way back in April – with Man United 1-0 up and pulled out a superb stop for the cameras from Sandro Ramirez’s drive.
And Mourinho, speaking after the game, was quick to make note of De Gea’s ability to spring into action after a prolonged period of doing next to nothing.
“It’s even more difficult when you don’t touch the ball,” he said. “You are in the game, you don’t touch the ball five, 10, 15, 25 [minutes], you don’t even smell the ball.
“When you are sleeping you don’t touch the ball for minutes and minutes and minutes and then have an important moment in the game. So it’s even more difficult and even more important. The ball goes there and you have to be ready for action it is even more difficult.
“So yes, he was important for us in a couple of occasions.”
Mourinho’s arrival at the club quickly coincided with the limelight moving away from De Gea as others further up the pitch took control.
But make no mistake: the Spaniard deserves to be regarded as United’s most important player – or at least one of them. Confidence is contagious and stems from the man between the sticks. A shaky goalkeeper, after all, confluences with a shaky defensive display.
And his intervention to prevent Rooney from writing Monday’s headlines, laying the platform instead for the home side to run riot, was an important reminder of the wider effect De Gea has on this United side.