United, the 20-time English champion in whose shadow City has lived for so long, was in City's Etihad Stadium fortress for the title clincher and derby. What could possibly go wrong?
Nothing in the first half. By halftime, City looked to have wrapped up the league title in record time with a 2-0 lead, shredding United apart with goals from captain Vincent Kompany and Ilkay Gundogan in a six-minute span. United was being embarrassed.
Then they swapped ends and it all went wrong. United scored three times without reply and won 3-2.
The home fans reacted as if the clock had been wound back two decades when City was relegated in successive seasons. "Typical City," as the faithful have long bemoaned with self-deprecating cries.
But this is a very different City — for a decade fueled by the wealth of oil-rich Abu Dhabi — though still capable of messing things up. When the weeping fans glance at the standings they will still see City 13 points clear of United at the summit with six games remaining.
It is inevitable Pep Guardiola will still get his hands on the silverware that has spent so long at Old Trafford, without City having to wait until the last day of the season like in 2012 and 2014. But this was a golden opportunity blown.
"This is tough mentally," Guardiola said. "We are sad for ourselves for our fans."
United counterpart Jose Mourinho was more irritated than sad when he addressed his players in the dressing room at the break.
"He said that we didn't want to be the clowns standing there watching them get their title," United defender Chris Smalling recalled.
For Paul Pogba, after an insipid first-half display, the ramifications of a defeat weighed heavily on his mind.
"If (City) won they are champions," the United midfielder said. "For all the (United) fans, it would be like death. To lose against City and to see them celebrate, I couldn't let that happen."
Particularly after a mischievous intervention on Friday from Guardiola, who claimed he was offered the chance to sign the Frenchman in the January transfer window.
His hair was dyed in City blue for the game but Pogba was very much a Red Devil.
Eight minutes elapsed in the second half when Alexis Sanchez's cross was chested down powerfully by Ander Herrera into the path of Pogba who tapped into the net.
City was stunned and didn't tighten up at the back. Within two minutes, United was level.
Pogba picked up possession around the halfway line and sent the ball out to Sanchez on the left. As the Chile forward held up the ball, Pogba raced into the penalty area, past Nicolas Otamendi to meet a cross and power a header into the net.
"If it's true that his agent offered him to some clubs," Mourinho said, "now the price has gone up."
City's fans who were singing "Stand up for the champions" at the start of the second half were momentarily stunned into silence.
Their defenders were as inert in the 69th minute when Smalling was left unmarked to run into the penalty area to meet Sanchez's free kick, volleying the winner past Ederson.
City had lost only once all season in the league at Liverpool, and been undefeated at home since the visit of Chelsea in December 2016.
But Guardiola, whose side was denied a penalty when Ashley Young flew into Aguero with a sliding tackle, rolled the dice and came up short. His starting lineup showed how Guardiola has prioritized lifting the European Cup — something United has done three times, while City has yet to establish its place among the continent's elite.
The principal attacking threats — Gabriel Jesus and Sergio Aguero — were left on the bench for more than 70 minutes. So, too, was player-of-the-year contender Kevin De Bruyne.
They are required on Tuesday to help overturn a 3-0 deficit against Liverpool to reach the Champions League semifinals.
The rested players weren't missed, in the first half at least. United crumbled, just like City did at Anfield on Wednesday.
And it was the team's longest-serving player who came through with the opener. Kompany was signed a week before the Abu Dhabi takeover in 2008 and he has been an integral part of the team's transformation, battling through more than 40 injuries in his City career.
When Leroy Sane swung in a corner in the 25th minute, Smalling was grappling at Kompany and still tugging at the defender's shirt as he leapt into the air. The Belgian was still able to head into the net.
Within 5 minutes and 15 seconds, the ball was in David de Gea's net again. Ilkay Gundogan swiveled and turned to meet Sterling's pass, then dispatched the ball into the bottom corner of the net.
United was barely able to get into opposition territory as City enjoyed the freedom to stroll across the pitch.
But Mourinho's reprimands in the dressing room had the effect desired as United denied City the chance to celebrate winning the title in front of its fiercest foes.
"Now we are going to try to see if we are brave enough to stand up again," Guardiola said, looking ahead to the visit of Liverpool.
- AP