Mourinho said he could "smell" a performance of this nature coming, complaining there had been a noticeable deterioration on the training ground since beating City.
"We couldn't play fast, we couldn't think fast, everything was complicated, " he said. It might seem that way, but as we've seen quite clearly lately, they are far from invincible.
City manager Pep Guardiola might have rubbed his eyes in disbelief at events at Old Trafford when he returned from his round of golf with his son.
The win was West Brom's first since a 2-0 success over Brighton on 12 January and only their fourth all season.
Guardiola, speaking after Saturday's 3-1 win over Tottenham at Wembley, declared his own satisfaction at the superb efforts of his squad this term.
West Brom, to their credit, also kept probing forward but they lacked the class required to really open United up at the back.
18 United should have a penalty when Ander Herrera goes down in the box under Craig Dawson's challenge but for some reason referee Paul Tierney says no.
Manchester City were always going to be favourites this year, a no-nonsense turnover of players and signings such as Bernardo Silva and Kyle Walker were never going to make them any worse but Manchester United fans expected their team to put up a bit more of a fight.
United remain second on 71 points, just a point ahead of third-placed Liverpool in the league table.
He also played 34 key passes - compared to Pogba's 25 - and provided six assists, which is a total of 40 chances created, still six more than the United man's record.
Afterwards, Mourinho indicated that his players did not execute the game plan as he had hoped. "One more touch to control the ball, one more flick, one more trick. There was no fluidity", he said.
United were thrilling at the Ethiad because they had to be.
Instead, the title was decided on a wet Sunday afternoon when the new champions were not playing. However, interim boss Darren Moore has at least restored some pride for the team heading into the Championship.
Sergio Aguero, who became City´s record goalscorer this season, added: "Ecstatic about clinching another title with this great team, and with all the support of our fans".
It was Arsenal's fifth consecutive away defeat in the Premier League.
City defender Benjamin Mendy, who missed most of the season due to injury, turned social media blue as he tweeted: "We are f*****g champions congratulations boys!"
Man United players look on dejected after the final whistle.