In order to prevent City from being crowned champions for a few more days, United had to avoid defeat against bottom club Albion at Old Trafford.
But Jay Rodriguez's 73rd minute header gave West Brom a stunning victory after they arrived at Old Trafford having lost nine of their previous 10 games.
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It is ironic that United came back from 2-0 down against City themselves last weekend to win 3-2 and delay City's party, only to surrender so tamely on home soil.
Mourinho claimed afterwards that he had not been happy after the City result because he knew they would win the league regardless and he accused some of his players of getting carried away.
"I don't know if I was the first or second or third but I congratulate them face to face when I had the chance," said Mourinho. "I knew that sooner or later they would be (champions) and if I was in their position I would be very upset if someone said they won the title because Manchester United lost against West Brom.”
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He added: "They won because they were the best team, they lost only a few points. Probably because I won eight championships, I was not in the moon with the victory against them. That doesn't make me feel super happy and in my opinion affected for sure some of the people that was too happy."
United's afternoon was symptomatic of their entire season.
Their squad is crammed full of wonderfully potent performers such as Juan Mata, Paul Pogba, Romelu Lukaku and Alexis Sanchez, yet the Premier League table, which shows them 16 points behind City, does not lie.
United have scored a full 30 goals fewer than City this season and Mourinho has not managed to get his side to click, despite being able to bring on further talent such as Marcus Rashford, Jesse Lingard and Anthony Martial.
Mourinho pulled no punches about his squad's lack of consistency, but the former Chelsea boss made it clear he believes he can return United to the summit of English football.
"I trust in my work, that is for sure - I have no reason not to trust," he said. “Eight titles are eight titles and three Premier Leagues are three Premier Leagues. I know how to win and I believe in myself. I think with my experience you don't win titles with inconsistency. You win titles with consistency at every level, not just the performance level but the mental level and the day by day. Everything must be consistent until the last day. One of the things we have to improve is our consistency."
The first half was a terrible advert for the English top-flight.
Jake Livermore had one decent effort saved by David De Gea and Lukaku smashed an effort right into Ben Foster's face but that was about it.
The second half was not much better but Foster did wonderfully well after 66 minutes to tip a Lukaku effort around the post before the visitors resistance was then unexpectedly rewarded with 17 minutes remaining.
Nemanja Matic panicked at the delivery of a Chris Brunt corner and he knocked the ball back inside to Rodriguez, who could not miss from three yards out.
After sacking Alan Pardew at the start of the month, Darren Moore has been handed the West Brom until the end of the season.
Although the West Brom board are believed to want somebody with more experience, Moore certainly did his chances of becoming the next full-time boss no harm, even if avoiding relegation looks a lost cause.
"West Brom is a club I care about a lot," he said. "I want to bring a family atmosphere to the club. Everybody is equal at the football club and we're all in it together."
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