To put all of this into context, perhaps the best place to start is with the man-of-the-match award. It went to a player, Ryan Donaldson, who house-shares with four of his Cambridge team-mates. The team languishing in 12th place in League Two had shown a wonderful spirit of togetherness and they were entitled to the long, raucous celebrations at the final whistle, notable for the lack of scramble to swap shirts with their illustrious opponents. At this level the players have to pay for their own replacements.

They were virtually encamped in their own half during the final exchanges when the pressure was almost relentless and Manchester United had enough of the ball in the last half an hour to think they could have avoided a replay. Yet it had been another scruffy display from Louis van Gaal’s players and it was perplexing to see how difficult they found it to demonstrate the gulf in quality against the team 75 places below them in the order of English football.

Richard Money had instructed his team to “look at the Manchester United shirt, look at that badge, then stick your chests out and go out and play” and Cambridge certainly showed great resolve in those moments when their opponents finally started to produce the fully joined-up football that might be expected of a Premier League side.

The home side were down to 17% possession midway through the second half but it was also true that they restricted the most expensively assembled group of footballers ever to visit this ground to only a couple of decent chances throughout the entire night.

It was a prodigious effort from a team that did not cost a single penny in transfer fees and honorary mentions should go to the two centre-halves, Michael Nelson and Josh Coulson, for their backs-to-the-wall resilience. The goalkeeper, Chris Dunn, was also picked out by his manager for acclaim.

Yet this was not merely a story of one team’s defiance and Money was not exaggerating when he said that they had been the more dangerous team in a first half when their opponents never settled. The outstanding chance of the night fell to Coulson and there were some bewildered looks when Van Gaal was asked afterwards to explain his team’s performance and said the referee, Chris Foy, was partly to blame without actually explaining what that meant.

What is beyond dispute is that four years ago Cambridge were one game away from slipping into Conference South. “This club has been in the doldrums for 10 years,” a jubilant Money reminded everyone, reflecting on a financial prize that was “incredible, massive, mind-blowing. I can’t even think of it.”

His team refused to be in awe of their opponents. Donaldson set the tone when he tried a nutmeg on Daley Blind inside the opening minute and, by half-time, there had been only flashes from the visitors to make it obvious that one team was three divisions above the other. Cambridge played as if utterly immune to stage fright.

Van Gaal had talked beforehand about suffering “twitching ass”, a modern-day version of squeaky-bum time, on the occasions he chose an orthodox back four rather than the wing-back system that has led to mutinous chants from the team’s supporters. Yet most of his twitching in the first half might have had more to do with his frontline on a night when Robin van Persie started among the subs and Wayne Rooney was given the night off.

Ángel Di María was particularly disappointing while Radamel Falcao and James Wilson both found it difficult to make a significant impact and there was also a sense of déjà vu for Van Gaal going back to his team’s laboured performance in their third-round tie at Yeovil. The same flaws were evident again and when a player of Di María’s refinement could be seen misplacing passes and putting the ball over the stand with one shot, the home side were entitled to think that the 11-time FA Cup winners might be vulnerable.

Cambridge also had seven six-footers among their outfield players and Money was off his seat after 31 minutes, convinced his team were about to take the lead when the ball dropped to Coulson from a corner. Coulson had inadvertently blocked the initial header from Nelson and was only a yard out when the ball came back to him among the pack of bodies. He could not twist his header on target and it was tempting to wonder whether Cambridge might eventually regret that moment when Di María and Michael Carrick finally started to pass the ball with more authority.

Cambridge had already started to lose their momentum by the time Carrick, with his first pass of real imagination, set up Falcao on the hour. Dunn turned the Colombian’s shot over the crossbar and Van Persie could not hit the target with a reasonable chance soon after replacing Wilson.

Cambridge were retreating further into their own half but they did also have a couple of breaks of their own during a tense finale and Money made a case afterwards that it was not just because of money that nobody was swapping shirts. “It’s a nice story but, honestly, none of them asked if they could anyway.” Cambridge had refused to be starstruck and Money was feeling generous. “We might buy them one each now,” he said.