Arsene Wenger's final match at Old Trafford as Arsenal manager ended in defeat as a stoppage-time header from substitute Marouane Fellaini earned Manchester United a 2-1 Premier League win on Sunday.
Paul Pogba had given United a 16th-minute lead before Henrikh Mkhitaryan, signed from United in January, brought Arsenal level, six minutes after the interval.
It looked like a youthful, mostly second-string Arsenal side had earned a respectable draw before Belgian Fellaini nodded in a cross from Ashley Young to seal victory for Jose Mourinho's side and secure their Champions League place for next season.
"OK, progress at many levels but not enough to be first instead of second and not enough to win the Champions League instead of the Europa League," Mourinho told reporters.
"But enough to finish second, to be stable to win some trophies to reach some finals but we need to do more," he added.
Wenger, who will leave Arsenal at the end of this season, has not tasted victory at United for 10 years and has never won away against a Mourinho team in the Premier League.
The Frenchman, who clinched the league and Cup double at Old Trafford in 2002, was centre stage before the game as he was presented with a gift by former United manager Alex Ferguson and Mourinho, and he was warmly applauded by the crowd.
But while the past was being generously acknowledged, Wenger also had an eye on the near future.
With a trip to Madrid to face Atletico on Thursday in the second leg of their Europa League semi-final, poised at 1-1, Wenger rang the changes, bringing in several fringe players and handing a debut to 21-year-old Greek defender Konstantinos Mavropanos.
It was the youngest side put out for a Premier League game by Wenger in nearly seven years but with the Europa League offering the only chance of Champions League football for his sixth-placed team, it was an understandable choice.
With no Mesut Ozil, Aaron Ramsey or Alexandre Lacazette, Arsenal were relying on their younger players and there were glimpses of promise.
But it was United who drew first blood when Pogba burst through the midfield and fed Romelu Lukaku whose cross was headed against the post by ex-Arsenal forward Alexis Sanchez and Pogba was on hand to convert.
It was a low key half, though, with little of the tension of some of Wenger's other visits to Old Trafford, although Mkhitaryan went close and United's Young struck the post with an attempted cross.
The hosts were mostly untroubled but lacked sharpness and they had a narrow escape just before the break when Reiss Nelson headed just wide.
The Gunners did get their equaliser, though, six minutes after the restart when Mkhitaryan was allowed to run at the United box before beating David de Gea with a well-placed low shot.
Fellaini was brought on to add height and physical presence for United and - not for the first time - that approach worked.
He headed against the post but Marcus Rashford's follow-in was ruled offside before United tried the same approach again, the Belgian rising to head in Young's cross and keep Mourinho's side firmly on course for a second-place finish.
They are five points ahead of third-placed Liverpool but 16 adrift of champions Manchester City with three games left.
For Wenger, there was satisfaction in seeing his young players compete against one of the top sides.
"It was unfortunate because we had a positive performance but we paid for the fatigue in the final 20 minutes from players who are not used to it. They did very well for the most part. It is unfortunate that they could not get the reward for that," he said.
