It would have been a goal worthy of winning any match, and no one is likely to forget the moment Amad Diallo announced himself as a Manchester United player, but no one, equally, could reasonably argue AC Milan did not deserve their late equaliser here.

For Diallo, it was a shame to see some of the gloss taken off his show-stopping turn only five minutes after his introduction as a second-half substitute, but, in truth, United did not do enough to win this game and must now go to San Siro next week with the tie carrying a very different complexion.

Simon Kjaer’s 92nd-minute header from a corner again exposed United’s enduring inability to defend set-pieces, with Nemanja Matic slow to track Milan’s goalscorer and blocked off in the process, and Dean Henderson making a hash of his attempted save.

After impressive showings against Crystal Palace and Manchester City, this was a step backwards for the England goalkeeper in his bid to snatch the United No 1 jersey from David De Gea.

It was not just Matic and Henderson that erred, though. Ole Gunnar Solskjaer paid the price for his strange decision to try to shut up shop and hang on to a fragile lead for the final 20 minutes. How heavy a price remains to be seen. “We go there knowing we need to score at least one, maybe two and get away with a draw,” the United manager said.

“It’s a header from six yards. He [Henderson] can save it of course. I have seen him save them but it’s the structure we lose. We dropped too deep. His [Kjaer’s] marker gets blocked off so he gets in front of him. We should have attacked the ball with the men we had there.”

Diallo’s moment of magic aside, there was just so little to recommend United, their shortcomings encapsulated in one dismal scene late in the first half when Harry Maguire missed an open net from a yard out, shinning the ball on to the inside of a post after Bruno Fernandes had flicked the ball to the far post from Alex Telles’s corner. “I should score,” the United captain said. “I won’t make any excuses for that.”

It was a jaw-dropping moment of a different kind to Diallo’s contribution, and the lowlight of a wretched first period from United who, simply unable to string a pass together, were outplayed by a youthful Milan side missing half-a-dozen regulars, including United old boy Zlatan Ibrahimovic. The Italians were particularly unfortunate that a fine volley by Franck Kessie in the 11th minute was harshly ruled out after Var decided the ball had taken the slightest touch off his left arm before he turned and shot.

These two clubs have 10 European Cups between them but this was a game that reminded you how far both have fallen.

“Stop giving the f------ ball away,” Maguire screamed after half an hour. United were crying out for some wit, a spark, anything to allow them to emerge from the drudgery of their careless, one-paced football. Enter Diallo for the injured Anthony Martial for only his third substitute appearance since completing an initial £18.2 million move from Atalanta in January.

Not even the eternal optimist Solskjaer would have believed the 18 year-old would make such an impact so quickly, and in such startling fashion, though.

It has evidently not taken Diallo long to learn that, if the run is right, Fernandes has the quality to find you with the pass and what followed was brilliant from the pair of them.

Stationed in between Fiyako Tomori and Diogo Dalot, facing his parent club for the first time since his loan to Milan last October, Diallo had the two defenders where he wanted them and suddenly made a dart in behind Tomori.

It was a superb run and Fernandes, spotting it, clipped an exquisite pass over the head of the on-loan Chelsea defender. Diallo twisted his body, craned his neck and then deliberately looped the ball over the advancing Gianluigi Donnarumma with the back of his head.

Diallo, mobbed by team-mates, celebrated by covering his right ear and left eye with his hands, the crying shame being that there were no fans present with which to share a moment that deserved an audience and a rapturous reception.

“The pass was incredible - that more or less made the goal,” Solskjaer said. “The flick is probably the only thing he can do and should do; perfect timing, perfect weight on the flick, a great moment for the kid.”

It was not enough on the night, though. United have it all to do in Milan. 

Good afternoon, will Cavani stay or go?

Ole Gunnar Solskjaer still hopes to convince Edinson Cavani to extend his Manchester United stay despite the striker's father suggesting he is ready to quit the club this summer.

After leaving Paris St Germain as their all-time top scorer, the 34-year-old free agent moved to Old Trafford in October and signed a deal until the end of the season with the option of a further year.

Cavani has scored seven goals in 25 appearances to date and Solskjaer has made no secret about his desire to keep him at the club next season, although the player's father has indicated that is unlikely.

Luis Cavani reportedly said there was a 60 per cent chance that his son would return to South America as he "does not feel comfortable where he is", mentioning Boca Juniors as a possible destination.

The Uruguay international subsequently posted a photo on Instagram of him playing for the club with the caption "proud to wear this shirt" - a jersey Solskjaer hopes he will don for a while to come.

"Well, Edinson's fully focused on being available for us and working really hard to get fit," the United boss said.

"From my bad Portuguese and his better English, he's very proud to play for Man United. He's always going to give his best as long as (he is here).

"If it's beyond this season as well, a decision hasn't been made. As we've said, he's made a great contribution so far.

"We're just working on getting him back on the pitch and fit - and we'll convince him when the weather in Manchester improves!"

Cavani is set to miss another United match on Thursday, when AC Milan arrive at Old Trafford for a mouth-watering Europa League last-16 clash.

Marcus Rashford is set to join the veteran on the sidelines for the first leg, having been forced off towards the end of Sunday's impressive 2-0 win against Premier League leaders Manchester City with an ankle issue.

"I'm not sure if anyone will be back, no," Solskjaer said ahead of facing Milan, who are able to play loanee Diogo Dalot against his parent club.

"David (De Gea) is not available. Donny (Van De Beek), no. Paul (Pogba), no. Edinson, maybe but probably not. Juan (Mata), no. And Marcus maybe but probably not.

"So, similar to Sunday but maybe we've lost Marcus.

"I don't think he'll be available for tomorrow. He's still getting treatment but he didn't join in the training today.

"Even though I'm normally a very optimistic guy, I still think he's out of tomorrow night and probably a doubt for the weekend."