DURING the first half, it had looked as though Cristiano Ronaldo was trying to win the Ballon D’Or rather than the match. In the second half, he went out and won the match.
There are theories that Ronaldo might not fit into the high-pressing manic work-rate ethos of United’s incoming interim boss Ralf Rangnick.
But while they still settle football matches on the basis of which team scores the more goals, rather than on running stats, there must surely be a starting place for one of the greatest goal machines of all time.
After receiving his work permit on the morning of the match, the experienced German Rangnick was up in the directors box giving his new charges a once-over before he arrives at the training ground tomorrow.
And he will have been relieved to see Ronaldo settle an entertainingly chaotic match with a second-half double that lifts United to seventh in the table, just three points off the final Champions League place - which must be the interim’s minimum target.
After being dropped by caretaker Michael Carrick for Sunday’s visit to Chelsea, Ronaldo had thrown a little strop about his great rival Lionel Messi being crowned World Footballer of the Year for a seventh time the following evening.
And early on, there were several failed attempts from Ronaldo at filling highlights reels with party tricks, like the good old days.
Now though the 36-year-old Portuguese is a world-class penalty-box predator and he twice put United ahead - the winner from the penalty spot - after United had fallen behind to one of the most bizarre opening goals you will ever see.
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Ronaldo was the only player on the pitch who can recall playing when United versus Arsenal was still THE fixture in English football - about a decade and a half ago.
Had he been left out of the starting line-up again it would surely have been a sign of the new Rangnick regime taking hold - however much United insist that the German wasn’t involved in the preparations for this match.
It was the first game at Old Trafford since Solskjaer’s sacking and they sang the Norwegian’s name long and hard on the Stretford End, as Rangnick took up his pew in the directors’ box.
After an early spell of Arsenal, Ronaldo received his first sight on goal and screwed a weak shot wide to the mocking jeers of the Gunners supporters - who spent much of the evening singing about winning the league here 20 years ago.
Then, the mayhem - and an incident which reminded you of one of those unlikely scenarios in ‘You Are The Ref’ cartoons.
As United defended a corner, Fred studded De Gea in the back of the heel, the keeper fell down in a heap on the goal-line and the keeper was still facing the wrong way as Harry Maguire cleared and Mo Elneny nodded down for Smith-Rowe to lash home.
Atkinson blew his whistle - seemingly out of sheer panic - just after the ball had crossed the line.
But during a lengthy delay as De Gea got to his feet, VAR Andre Marriner ruled - quite rightly - that there had been no offence and the goal was given.
Arsenal celebrated wildly, to the sound of booing around Old Trafford, and there was clearly no chance of them sportingly waving United through for an equaliser.
Soon after, Ronaldo attempted a bicycle kick only to catch Ben White on the shoulder, and White then appeared to push over Alex Telles for a decent penalty shout. Atkinson might have looked to even things up but, admirably, he didn’t.
Gabriel Martinelli dragged a shot wide when a simple cut-back to Smith-Rowe would surely have brought a second Arsenal goal.
And then United levelled two minutes before the break. White ceded possession to Sancho who darted forward and fed Fred, whose artful reversed roll of the studs set up Fernandes to ram into far corner - although keeper Aaron Ramsdale might have kept it out.
After De Gea saved well from a corner which cannoned off Gabriel’s back, Ronaldo produced a saucy turn which filleted Thomas Partey but a shot which was blocked by Ramsdale’s body.
He did not have to wait long to get on the scoresheet though, Rashford advancing down the right and squaring for Ronaldo to sweep it home from six yards.
Arsenal’s response was almost immediately, a diagonal pass from Partey releasing Martinelli who cut back for Odegaard to equalise with a sweet first-time shot into the far corner.
A sweeping pass from White sent Pierre-Emerick Aubameyang striding clear but after a poor first touch his shot was clawed away by De Gea.
Ronaldo’s winner needed the assistance of VAR Marriner, Odegaard wrapping his legs around Fred in a late lunge which Atkinson failed to spot.
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The man in the No 7 shirt - ninth in the Ballon D’Or ranking - won’t have earned any great individual plaudits for ramming the spot-kick straight down the middle.
But he earned his side three points - and was handed a standing ovation when he was withdrawn straight after a lung-busting 80-yard dash in the 88th minute.
See, Ralfie, he can run, this fella. And he can definitely score.