Dean Henderson was only a minute away from another clean sheet, his tenth in his first 16 appearances for Manchester United and third in three games since stepping into the starting line-up for David de Gea.
Another would only have strengthened the ambitious young goalkeeper’s case to be United’s long-term, first-choice, but then came Simon Kjaer’s header.
Henderson was caught slightly underneath it, and though he got a touch on the ball, he could not keep Milan’s late equaliser and away goal out. The dynamic of the last-16 was turned on its head. United’s Europa League progress could hinge on that moment.
Attention immediately fell on whether Henderson could have done better, such is a goalkeeper’s lot in life.
That pressure to perform is even greater when you are attempting to dislodge a No 1 of De Gea’s pedigree. The scrutiny is that bit more intense. The spotlight is that bit brighter.
Yet Henderson has coped well with that pressure up until now and has responded admirably to mistakes in the past. It was telling that Ole Gunnar Solskjaer refused to criticise his goalkeeper after the final whistle, instead pointing the finger at his defence and familiar failings on set-pieces.
De Gea is now back in Manchester after attending the birth of his first child but has not trained this week while self-isolating in accordance with government travel restrictions.
It is likely that Henderson remains in the starting line-up for the visit of West Ham this evening, then come Europa League and FA Cup engagements in which United’s second-choice goalkeeper would normally play.
Beyond that, nothing is certain. There have been no promises or guarantees and Henderson was not told that he would be given a specific number of games to stake his claim while De Gea was away.
He has not received any assurances that he will keep his starting place now De Gea has returned, which is not to say that he will immediately lose it either.
It simply means that, for all that has changed over the past couple of weeks, United’s goalkeeping situation remains largely the same: both players will have opportunities to play before a final decision is made at the end of the season.
When that decision arrives, it will be one of the most difficult calls that Ole Gunnar Solskjaer has had to make during his two-and-a-half years in the job. It could go a long way in shaping the next two-and-a-half years, too.
That is why Solskjaer needed a season to make sure he got it right.
Though the current dynamic is unorthodox, it is one that the United manager wanted. Solskjaer is a firm believer that healthy competition for places brings out the best in players. It may even be preferable when making a decision as important as this.
Henderson is said to have been more than happy to go head-to-head with De Gea last summer, turning down interest from the Premier League and abroad to sign a new five-year contract which increased his salary to the level of a starting goalkeeper at a top club.
De Gea, of course, is the best-paid goalkeeper in the world but did not rest on his laurels in the face of this new competition.
United’s No 1 revised his training methods with United’s goalkeeping coaches in the early part of the season, specifically in response to the challenge from Henderson. The questions over his starting status briefly went away as his performances improved.
And for all the talk of De Gea’s decline, he remains one of the best in his position in several areas. Few goalkeepers in the world can, for example, boast his supreme reflexes.
There was a timely reminder of that at Stamford Bridge recently with a save from Hakim Ziyech which was not only impressive for the speed of his reactions but the strength of the hand which denied a likely goal.
That instinctive, natural ability may never leave him. The issue, though, is that the consistency he last showed three years ago has now eluded him and familiar limitations in his game have repeatedly cropped up.
Whether it is distribution, communication, command of his area or coming to collect crosses, more and more aspects of De Gea’s game have come under the microscope.
There have been important saves in recent meetings with West Bromwich Albion and Chelsea but then in other games, like the 3-3 draw with Everton, a goalkeeper that once earned his side points is now costing them just as many.
It does not help De Gea’s cause that his understudy has impressed in several of these problem areas.
Henderson is certainly a vocal presence, whether that is through encouragement or constructive criticism, and that is necessary in a team that is relatively quiet outside of captain Harry Maguire.
On Real Sociedad’s visit to an empty Old Trafford, he was heard telling Axel Tuanzebe to talk to his team-mates more and called on others to help the defender out when he was in possession.
Later that same evening, the first words which Shola Shoretire heard as he became United’s youngest-ever player in European competition were ones of encouragement from Henderson. “Come on Shola,” he yelled from 50 yards away, “express yourself!”
Statistical comparisons of De Gea and Henderson’s performance are difficult due to the disparity in their playing time.
Even so, they tend to favour Henderson. The England international has collected a greater percentage of crosses - a league-average 7.3 per cent, compared to De Gea’s low 4.4 - in his minutes on the pitch.
Take Kjaer’s equaliser out of the equation and his shot-stopping has largely impressed, too. Post-shot expected goals is a statistic which measures the quality of a shot on target, allowing us to estimate how likely a goalkeeper is to save it.
According to this measure, Henderson is preventing quality chances at a similar rate to Aston Villa’s Emi Martinez and Fulham’s Alphonse Areola, two of the stand-out goalkeepers in the Premier League this season. De Gea, by contrast, comes in at league average.
One memorable example of such shot-stopping abilities was the late save to deny Patrick van Aanholt in the goalless draw at Selhurst Park, but then some say that type of 'clutch' save is a deliberate product of his career path.
Loans at Sheffield United and, before that, Shrewsbury Town often saw Henderson playing in relatively low-scoring sides, making his performances at the other end all the more important.
Only five of the Blades’ 14 Premier League wins last season came by two goals or more and their survival bid was helped by 12 draws.
Even his distribution, not previously thought to be one of his strong suits, is improving.
After spending two years largely kicking it long at Bramall Lane, Henderson returned to Bramall Lane in December and made an error while attempting to play out of the back, causing United to go behind.
Yet Henderson looks more comfortable with the ball at his feet, launching fewer of his goal kicks than De Gea over the course of the campaign, and was happily playing it short at the Etihad last Sunday.
That is not to mention the remarkable, Schmeichel-esque throw to Luke Shaw to create an opening for United’s second goal in the derby out of nothing.
Perhaps the strongest argument in Henderson’s favour is that he is providing genuine competition to a goalkeeper with the pedigree of De Gea, who has made 434 appearances, won a Premier League title and been named United's player of the year award four times.
Perhaps the best comparison, therefore, is not between Henderson and present day De Gea, but Henderson and the young, uncertain De Gea who arrived in Manchester in 2011, took a while to adapt, yet still went on to become a club great.
Could Henderson, who until Thursday night had made a smoother start, do the same?
Solskjaer will take stock of each player’s performances and consider all of this between now and the end of May. Of all the possible outcomes, one thing is certain: the current situation is unsustainable in the long-term.
Whatever decision Solskjaer reaches, something will change. There is next to no chance of De Gea continuing to play United’s Premier League and potential Champions League games next season while Henderson only plays in the cups.
Henderson does not consider himself to be a second-choice goalkeeper. De Gea has not regularly warmed a substitutes’ bench since breaking into Atletico Madrid’s starting line-up more than a decade ago.
United, essentially, have two players who expect to be first choice this time next year.
Whoever becomes the undisputed No 1 will remain at Old Trafford. Whoever is second-choice will be ready to look for opportunities elsewhere.
The next few weeks will be critical when it comes to resolving the biggest dilemma of United’s season with its biggest decision.
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