The problem with football, as Arsène Wenger is rapidly finding out, is that there is never any guarantee of a happy ending. Wenger had been clapped to his seat but the hospitality did not extend to the pitch. His team had conceded a 91st-minute winner and, however rich the tributes might be for the departing manager, Arsenal have now lost six successive away fixtures in the league for the first time since Billy Wright was their manager in 1966.

Wenger left Old Trafford with a silver vase, presented by Sir Alex Ferguson, and the lingering memory of this game will be the applause that followed the Frenchman along the touchline before the start. By the end, however, Old Trafford had reverted to type. “We want you to stay” was the cry, directed towards Wenger, after the stoppage-time header from Marouane Fellaini that meant Arsenal’s manager will remember his final visit to this stadium with little affection.

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That goal means Arsenal are still the only team in England’s top four divisions to have lost every league game on the road since the turn of the year. It is an ignominious record and typical, perhaps, of the club’s decline if it is generally accepted that the teams with the better away results are usually the ones with the stronger personality. Arsenal, for example, rarely lost on the road in Wenger’s peak years.

Those days can feel a long time ago when it is Alex Iwobi in central midfield, rather than Patrick Vieira, and Calum Chambers operating in the space Tony Adams used to occupy. Yet it would be difficult to be too harsh when Wenger’s team selection was guided by the fact that the team have the second leg of their Europa League semi-final coming up against Atlético Madrid on Thursday.

Mesut Özil, Aaron Ramsey, Alexandre Lacazette and Jack Wilshere were among the players given a day off. All the same, Arsenal held their own for long spells and, at times, threatened a winner of their own following Henrikh Mkhitaryan’s equalising goal against his former club.

In total, Wenger made eight changes from the team that drew against Atlético last Thursday. This was the youngest Arsenal team, with an average age of 24 years and 67 days, since the infamous 8-2 defeat here in August 2011 and to put it into perspective, there were three teenagers on the bench – Eddie Nketiah, Jordi Osei-Tutu and Joe Willock – whose shirt numbers were in the 60s. Of Arsenal’s entire match-day squad, Mkhitaryan was only player with a number between one and 11.

Wenger’s record against Mourinho – now two victories from 19 attempts – is fairly lamentable even with his strongest group of players. Here, though, he was trying to beat his old rival with the kind of inexperienced team he would usually put out in the Carabao Cup. Konstantinos Mavropanos, a 21-year-old Greek defender, became the 200th player of the Wenger era to be given a debut. Arsenal’s midfield included Reiss Nelson and Ainsley Maitland-Niles and, with so many changes for the visitors, it must have been troubling for Mourinho that his team could not exert more control. Wenger summed it up accurately as “a positive performance but negative result”.

Alexis Sánchez certainly found it heavy going at times against his former club, whereas Maitland-Niles, in particular, seemed determined to show that Arsenal have more strength in depth than many people might realise. Sánchez was, however, prominently involved in the opening goal for United, in the 16th minute, when Romelu Lukaku’s cross picked him out at the far post. Sánchez’s header was going in until Héctor Bellerín thrust out a leg to divert the ball against a post. The rebound fell kindly for Paul Pogba and he hooked the ball into an exposed net.

All that had originated from Nelson, Arsenal’s youngest player losing the ball to Jesse Lingard close to the halfway line. It was a lesson for the 18-year-old that, at this level, those kind of mistakes tend to be punished. Yet the teenager did well overall and the build-up to Arsenal’s equaliser was a reminder that even players of considerable experience can be guilty of costly lapses.

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For this one it was a combination of errors, with Nemanja Matic and Ander Herrera leaving a pass from Antonio Valencia to one another and as soon as Granit Xhaka sprung the counter-attack, United were vulnerable. Mkhitaryan had sprinted upfield in support and his low shot from just outside the penalty area picked out a gap between Victor Lindelof’s legs to find its way into David de Gea’s net.

By that stage, United had lost Lukaku to a foot injury and it was not until the final exchanges that the home side started to pin back their opponents. Fellaini hit a post with one header and when the substitute Marcus Rashford bundled in the rebound the goal was ruled out by a late offside flag. Unfortunately for Wenger, the next time Fellaini won a header in the penalty area there was no intervention.

That goal should confirm United’s second-placed finish, five points clear of Liverpool in third with a game in hand. Arsenal, on the other hand, have lost 10 away fixtures for the first time since the 1989-90 season and, with 57 points from 35 games, are guaranteed to finish with the worst total of Wenger’s 22-year reign, the previous low being 67 in 2005-06. He talked afterwards of his gratitude for the “classy” gift from United and the crowd’s applause. But he must have preferred the days when the crowd in Manchester had cause to fear his teams. “It shows that once you’re not a danger any more, people love you.”