It was, indeed, a day to underline how quickly fortunes change in football.
For Mourinho, it was a happy return to Leicester 14 months after the Foxes brought an embarrassing end to his second Chelsea reign.
For Leicester it was another miserable chapter in their increasingly woeful title defence.
A year ago today Claudio Ranieri’s men won at Manchester City to move five points clear at the top of the table. Today they sit a point outside the relegation zone.
And for United themselves, it was the perfect way to answer Mourinho’s pre-match mind games.
On Friday, Mourinho had called on his players to step out of their ‘comfort zone’.
By last night he was praising their ruthlessness after goals from Henrikh Mkhitaryan, Zlatan Ibrahimovic and Juan Mata put the faltering champions to the sword.
“The first chance we missed but the second, third and fourth we scored,” said Mourinho.
“We didn’t play better than we did in many other matches. We played solid. We controlled the game and we created chances but we scored goals and that makes a difference.
“My team is playing very well but for many years in my career, especially in this country, when my teams were ruthless and when my teams were phenomenal defensively and very good on the counter-attack, I listened week after week and that was not good enough in spite of winning titles many times.
“I’m at Manchester United and I don’t want to change the profile of our play. This is the way the fans and the club want us to play but I also don’t want to be the manager of a team that plays very well, creates a lot of chances and doesn’t win matches.
“So we need to score goals and today we did.”
Leicester’s start was encouraging with Riyad Mahrez cutting in from the right and sending a cross-shot just wide of the far post in a rare flash of the dynamism that has so often been missing this term.
A Mahrez free-kick moments later that gave United a further scare, with Jamie Vardy’s flick-on towards the far post forcing Henrikh Mkhitaryan into a hurried interception. But United ought to have led when Mkhitaryan found space on the right and crossed for an unmarked Marcus Rashford, who blazed over.
Juan Mata was a shade fortunate to escape sterner punishment when he was booked for a horribly timed lunge into Vardy’s ankles.
United should have led when the previously quiet duo of Zlatan Ibrahimovic and Paul Pogba combined to tee up Rashford, who drew a smart save from Schmeichel. But there was no stopping them in the late first-half burst that put them in total charge.
First Mkhitaryan skipped past Robert Huth as the Leicester defender committed himself, raced away from Morgan and planted a low shot in off the body of Schmeichel.
Then Valencia burst down the right and crossed for Ibrahimovic, who stepped away from Morgan to finish comfortably.
United’s third goal took just four minutes of the second half to arrive as Mata fed Mkhitaryan and capitalised on Leicester’s defenders switching off to collect the return pass and beat Schmeichel.
It might have got worse for Leicester moments later but Schmeichel made a smart low save to deny Rashford.
And there was more chaos in the Leicester defence as Mata sprang their horrid attempt at an offside trap and only spared them more punishment with a poor touch that allowed Schmeichel to gather.
United relaxed into a comfortable task of preserving their lead but they threatened a fourth goal late on when Valencia teed up Mkhitaryan, who fired over.
“For 40 minutes we played OK then after that it was unacceptable,” said Schmeichel. “It is not good enough and sums our season up at the moment.
“Let’s not talk about last season. It’s gone. We need to stand up and be counted. There are a lot of factors that go into it but this season has not been good right from the start.
“It is time for each one of us — from the top to the bottom of this club — to be counted. If we don’t we will be relegated.”
Schmeichel 5; Simpson 5, Morgan 5, Huth 4, Fuchs 5; Mahrez 5, Drinkwater 5, Ndidi 7, Musa 5 (King HT, 5); Okazaki 5 (Gray HT, 6); Vardy 6.
Chilwell, Albrighton, Kapustka, Zieler, Benalouane. Booked: Drinkwater, Fuchs.
De Gea 6; Valencia 7, Bailly 6, Smalling 7, Rojo 6 (Blind HT, 6); Mata 7 (Fellaini 77), Herrera 7, Pogba 7, Mkhitaryan 8; Ibrahimovic 7, Rashford 7 (Young 83).
Martial, Lingard, Carrick, Romero. Booked: Mata, Herrera, Pogba, De Gea.