Pep Guardiola’s defending champions were coasting at 3-0 with 20 minutes to go thanks to Bernardo Silva, Riyad Mahrez and an Andreas Pereira own goal after embarrassing their hosts with a display of dominant, mercurial football at Old Trafford.
However, one lapse allowed Rashford to score the goal which made it 3-1 to give them a chance in three weeks’ time, having won at the Etihad barely a month ago in the league.
A bullish Ole Gunnar Solskjaer had said pre-match they still thought they were the biggest club in the world but this performance showed just how far they were from that.
His City counterpart’s decision to leave his two recognised strikers on the bench and start Silva as a false nine raised a few eyebrows but it soon became apparent this was to be one of those Guardiola masterclasses.
It was a performance reminiscent of his Barcelona in their pomp, when Lionel Messi was given the freedom of the Nou Camp to orchestrate proceedings from anywhere on the pitch.
In 45 minutes Silva scored one and had a hand in the other two as he roamed freely, meaning City’s forward line resembled an expensively assembled carousel as Raheem Sterling, Mahrez and Kevin De Bruyne all rotated in the central role, leaving United’s centre-backs Phil Jones and Victor Lindelof chasing shadows.
The fact they often had no one to mark left them alone with their thoughts and the dilemma of whether to stay or go to and find an opponent but while they pondered that City ran riot.
United actually began the brightest with Rashford racing past a statuesque Nicolas Otamendi only for his cross to be cleared by Fernandinho, who could have conceded a penalty moments later had the United forward opted to go down under his challenge rather than stay on his feet and settle for a corner.
But in an instant the match, and effectively the tie, changed in an instant. Out of nowhere Silva fired one into the top corner off the underside of the crossbar from 20 yards and City never looked back.
United, however, were in danger of getting whiplash as they tried to comprehend the whirl of blue shirts which buzzed around them.
Silva, dropping deep on the right, turned creator as his exquisite through ball invited Mahrez to run through the middle, and although Lindelof should have done better in cutting it out, the Algeria international calmly rounded David De Gea and rolled the ball into an empty net.
The third goal was a classic counter-attack as when United lost possession on edge of City’s box Silva picked out Mahrez, who in turn found De Bruyne.
Jones, starting his first match since being hooked at half-time against Sheffield United in November after injury ruled out Harry Maguire, was ignominiously put on his backside by the Belgium midfielder, whose shot was diverted by De Gea into the path of Pereira, who could not stop the ball rebounding over the line.
“Ole Gunnar Solskjaer, we want you to stay” was one of the kinder chants from the visiting City fans.
It was a tough watch for former United team-mates Wayne Rooney, Nicky Butt and Ryan Giggs, sitting together in the directors’ box, but much more enjoyable for City’s Oleksandr Zinchenko watching in the away end.
City had other first-half chances for Silva and Sterling while United’s ‘best’ effort saw Mason Greenwood bundle a shot wide.
United offered slightly more forward motion after the break and were rewarded with Rashford’s 70th-minute goal when Rodri lost possession on halfway and the England striker ran through the inside-right channel and stroked a shot across Claudio Bravo.
City suffered another blow when De Bruyne was forced off after being laid out in an aerial collision with Lindelof and things could have been worse, with Bravo allowing Rashford’s long-range shot to slip through his grasp with the ball dropping wide of a post.
United somehow dodged a bullet with their second-half performance but still have plenty to do if they are to turn things around.