The FA Cup is many different things to many different people: magic, romantic, lucrative, overblown, exciting, boring. It is an important, prestigious competition while you’re in it, and a distraction that has long lost its lustre when a 41-year-old part-time plumber sends you tumbling out in front of 2,000 fans.

There is a reason it has a reputation as the great leveller, providing numerous Davids the means with which to topple many a comparative Goliath. It narrows the widest of gulfs, balances the most uneven playing fields and shortens the most tempting of odds.

Watching Manchester United hold Chelsea at arm’s length on Monday, pausing only to draw them close enough to pick them off twice, made this feel like a classic cup mismatch. The current difference between these two sides seems far greater than the one point that separates them in the Premier League, or even the two goals that set them apart here.

It was not quite United’s “best performance of the season”, as the typically understated Martin Keown had it. Nor was it even Chelsea’s worst game against a Manchester-based side in the past eight days. But it did highlight their respective shifts in momentum: the visitors started their season from the bottom yet are now racing up the stairs, while the hosts reached the top floor early before hurtling down in a seemingly broken elevator.

If Zinedine Zidane is as wise as his Champions League-laden medal collection suggests, his list of demands to take over at Stamford Bridge will include more than three vague points about renewed contracts, more money and increased influence. Chelsea need an entire personality transplant, and surgery more invasive than cosmetic.

The Blues have to be ruthless. Marcos Alonso, who lost Ander Herrera for the first goal, should not be a regular starter. Cesar Azpilicueta, bullied by Marcus Rashford at one point in the second half, can no longer shoulder the defensive burden alone. He’s knackered. Jorginho, anonymous throughout, similarly cannot carry the midfield weight. Kepa’s ability to get a hand to any low shot but still not prevent it from crossing the line has to be addressed. And how much magic can Eden Hazard be expected to conjure without a capable assistant?

Maurizio Sarri has been unfairly painted as something of a chancer by a media desperate to cast its pantomime villain, but as time passes you fear Roman Abramovich will soon descend from the stands to greet him with a custard pie as the fans shout “He’s behind you!”.

As results and performances continue to deteriorate, he is doing little to fight against that tide. The collective groan could be heard reverberating around the stadium when Willian replaced Pedro on 58 minutes and Ross Barkley took Mateo Kovacic’s place after 70, such was the painful predictability of it all.

It does not help that Ole Gunnar Solskjaer is achieving much more despite boarding a sinking ship mid-season. Those (named Paul Ince) who suggest anyone could have enjoyed similar success simply by not being Jose Mourinho must recognise that the Norwegian has won more games away at Big Six opposition (three in three matches) than the Portuguese ever did as United manager (two in 14 matches).

This was far more difficult a task than simply regaling his squad with stories of ‘the United way’ and how he won the Treble under Sir Alex Ferguson. Solskjaer was robbed of Jesse Lingard and Anthony Martial, two of the most influential players in his counter-attacking system, by injury. The fear was that he would try to crowbar Alexis Sanchez and Juan Mata into the same formation; the reality was that Mata sat behind a roaming Rashford and Romelu Lukaku to shadow Jorginho and pull the attacking strings.

It worked perfectly, giving Paul Pogba the licence and freedom to make Stamford Bridge his own playground. The Frenchman assisted the brilliant Ander Herrera with a sumptuous cross from the left-hand side for the opener, then set Rashford haring down the right before finishing off the move for a fantastic second.

Even during Chelsea’s most concentrated period of pressure, they never looked like breaching Sergio Romero’s goal. Both of their shots on target came in the 11th minute, with Luke Shaw, Chris Smalling and Victor Lindelof’s dogged and determined defending only a partial explanation.

That United had an idea and the hosts did not should be most damning. They imposed themselves early, exploited the chinks in an armour that is starting to fall apart, and shielded themselves against Chelsea’s passing of wind instead of having to weather any subsequent storm.

Perhaps the difference was that Solskjaer is managing in the now, while Sarri is working towards the future; the Italian would certainly suggest his task is more difficult, succeeding a man whose last game was to win this very competition, rather than taking over from a doomed dictator. But Solskjaer is the short-term coach with better long-term prospects than the project leader.

The fact is that Solskjaer adapted to a onerous situation, while Sarri continues to show he might well be too stubborn, too averse to change, to survive long enough to enact any grand plan. One found the diamond formation at a time when the other is struggling to even get out of the rough.

Matt Stead