Marouane Fellaini headed a stoppage-time winner to earn Manchester United a 2-1 home victory in Arsene Wenger's last visit to Old Trafford as Arsenal manager.
Wenger, presented with a gift by old rival Alex Ferguson before kick-off, rested the likes of Mesut Ozil and Alexandre Lacazette with an eye on Thursday's Europa League semi-final, naming his youngest league XI since 2011's infamous 8-2 loss at United.
And the Frenchman's young side were on track to do him proud, Henrikh Mkhitaryan levelling on his Old Trafford return after Paul Pogba's early opener for United.
But although United's attack was depleted by an injury that forced Romelu Lukaku off early in the second half, Jose Mourinho's men came up with a later winner.
Ashley Young's inswinging cross from the left was perfect for Fellaini to rise highest in the box to flick home the decisive goal in the second minute of added time.
A sixth successive defeat away from home in the Premier League will be irrelevant if Wenger can inspire a tunraround away to Atletico Madrid as United tighten their grip on second place in the table.
FT: 2 Arsenal 1. wins it at the death for United - what a finish! April 29, 2018
Pogba failed to take advantage of an early chance but he made amends with the opening goal in the 16th minute.
Lukaku floated a cross to the back post and, although Hector Bellerin deflected Sanchez's header on to the post, Pogba was well placed to cushion in the rebound with a cushioned volley.
Like Sanchez, Mkhitaryan was facing the club he left in January, and the Armenia international almost levelled with a drilled strike that flew just wide.
Mkhitaryan and Maitland-Niles had penalty appeals rejected as United's intensity dropped, allowing Arsenal into the game, and Pierre-Emerick Aubameyang sent a poor header straight at David de Gea in the 36th minute.
Ashley Young's mishit cross rattled the post as United rallied at the end of the half, but the game had a strangely low-key feel, with little riding on the outcome.
Lukaku limped off early in the second half after taking a knock to his ankle and Mkhitaryan - who refused to celebrate - promptly levelled the scores in the 51st minute.
United allowed their former player too much space in midfield, Mkhitaryan finding the gap between Victor Lindelof's legs to beat De Gea from the edge of the penalty area.
1 - Henrikh Mkhitaryan is the first player to score for and against Manchester United in the same Premier League season. Devilish. April 29, 2018
A raft of second-half substitutions saw an already low tempo fall even further, although former United striker Danny Welbeck stung De Gea's palms with an ambitious strike from range and Fellaini rifled a drive over the crossbar.
United thought they had scored a late winner when Rashford bundled in on the line after Fellaini's header rebounded off the post, but the offside flag cut short their celebrations.
There was no stopping United in added time, though, Fellaini glancing in an excellent header from Young's perfect centre to ensure Wenger would only leave Old Trafford with his present from Ferguson.
