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.
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," Mourinho said.
"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.
"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.