Arsene Wenger’s final trip to Manchester United as Arsenal manager ended in an agonising defeat after an injury-time winner from Marouane Fellaini.
The Belgium international flicked a header past Gunners goalkeeper David Ospina from an Ashley Young cross.
Just three minutes earlier he had headed against the post, with Marcus Rashford bundling home the rebound from an offside position.
United had opened the scoring on 16 minutes when a header from former Arsenal player Alexis Sanchez was knocked on to the post by Hector Bellerin and Paul Pogba stabbed home the rebound for his third goal in four league games.
The hosts lost striker Romelu Lukaku to injury early in the second half, and moments later Henrikh Mkhitaryan was left in plenty of space to weave and strike through the legs of Victor Lindelof.
The game’s tempo petered out after the equaliser – until Fellaini – who Jose Mourinho says is close to a new contract – popped up to snatch the win for Manchester United, and secure Champions League football for next season.
Wenger opted to make eight changes to his team, with one eye on Thursday’s Europa League semi-final second leg at Atletico Madrid, which is poised at 1-1.
With an average age of 24 years and 67 days, it was Arsenal’s youngest starting XI in the Premier League since August 2011 – in their infamous 8-2 defeat at Old Trafford.
It was a different story this time around. The Gunners made few mistakes and looked lively in parts, with Ainsley Maitland-Niles and Danny Welbeck testing United keeper David de Gea.
Mkhitaryan produced a cool and accurate finish for his goal, before being taken off with a knee injury which will be assessed before Thursday.
But three defenders failed to deal with a rising Fellaini in the box as he headed home a winning goal that meant Arsenal remain the only team in England’s top five tiers without an away win in 2018.
“The players are destroyed because they gave everything and were caught in the final minute,” said the Gunners boss.
“There were many question marks if we would be strong enough to fight but the quality was good the spirit was good, many players stood up. It was a negative result and a positive performance.”