The host threw everything at West Brom in the closing stages, but were unable to find a breakthrough that would have delayed City's title confirmation until at least Wednesday when United take on Bournemouth.
However, City were finally crown Premier League champions when United suffered a stunning 1-0 defeat at home to bottom-of-the-table West Brom.
United now find themselves sat 16 points behind City with four games to go.
Will United now go on to win the FA Cup?
He dismissed talks of being carried away following last weekend's derby win against Manchester City.
The Baggies stunned the Red Devils courtesy of an opportunistic second half goal from striker Jay Rodriguez, thus confirming Manchester City's status as Premier League champions. The story's only one: "how many points they won".
'I saw lots of people on the moon because we won against Manchester City.
"City were the best team, sooner or later they would get the points, so no dramas when the best team wins the league".
Prompted by the abject nature of the opening 45 minutes, Mourinho brought on Jesse Lingard at half-time and he was immediately in the heart of the action as United attempted to wake from their slumber.
Jose Mourinho has confirmed that Manchester United will not do "anything crazy" in the summer transfer window as he looks to strengthen his squad.
"I trust in my work, that is for sure", added Mourinho. "Eight titles are eight titles and three Premier Leagues are three Premier Leagues", he said. I believe in myself.
Their fans, who had celebrated the comeback victory at the Etihad last week, booed Jose Mourinho's side off the field after they lost to a team who had not won since January.
"You do not win titles with inconsistency, you win with quality". Everything must be consistent until the last day. "We keep fighting one game at a time". For instance, he created an excellent chance for Romelu Lukaku, but Ben Foster made a de Gea-like save, stretching out to his side.
Jay Rodriguez pounced on a bad clearance from Nemanja Matic following a corner kick in the 73rd minute to hand the Baggies a much-required lead. Huddersfield, Newcastle, West Brom; these are some of the teams against whom United have lost this season and it is unsurprising to see them so far off the top spot at the moment. With West Brom nearly set to be relegated even after this victory, it begs us to question whether a player of Rodriguez's character deserves to be relegated too. "There is a wonderful belief at the club and we showed that today".