It wasn’t quite a match that lived up to being Arsène Wenger’s last against his most bitter rivals in English football.
Marouane Fellaini’s goal at the death stole away what would have been a largely deserved draw, after Henrikh Mkhitaryan’s 51st-minute strike cancelled out Paul Pogba’s deft first-half goal.
There had even been a rare, but very pleasant, show of cordiality from Sir Alex Ferguson pre-match as he presented his long-time foe with a gift at the side of the pitch.
Here are five things we learned from the match, as Old Trafford bids adieu to Arsène.
Route One saves United blushes
Mourinho’s side had faded after the break and only really recovered in the final ten minutes, finally producing chances and their winning goal.
Prior to Fellaini’s header in the 91st minute, they had failed to manage a shot on target in the second half and allowed Arsenal back into the match with some lax defending.
Nemanja Matić and Ander Herrera watched as the ball rolled past them to Granit Xhaka, who laid it onto former-United man Mkhitaryan to fire into the bottom-left corner from the edge of the box.
They’d been stifling Arsenal for the most part, who completed just nine passes in the final third after having made 45 in the first half, but it wasn’t quite as convincing as the stats suggest.
True to form, Fellaini's introduction saw a third of the passes he received coming from crosses, two of which led to shots.
Perhaps they were content with a draw. It would have meant that they would be nine points ahead of fifth-place Chelsea with three games to go, but the last-gasp win ensures their Champions League status for next season.
Arsenal stopped messing around
Graeme Souness vented his ire at Xhaka at half-time for a silly decision to slide in in the lead-up to United’s goal, and for no Arsenal players having a go at the midfielder for it.
The ‘lack of leaders’ trope is a well-worn, and quite possibly correct, criticism of Arsenal, but there was another latter-era Wenger cliché on sight in the first half too.
Arsenal got to the opposition’s box quite frequently and completed seven passes into it but could only produce three shots inside the box (2.33 in-box passes per in-box shot).
Contrast that to United in the first half, who had five passes inside the box but four shots in the area (1.25 in-box passes per in-box shot).
After half-time, Arsenal struggled to get that far up the pitch very often. However, they took sight of goal when they did.
They only completed one pass into the box, for two shots inside the box (0.5 in-box passes per in-box shot), with Mkhitaryan’s goal also an example of catching the opposition unawares instead of searching for perfection.
Sánchez-Mkhitaryan swap makes more sense than ever
Watching Alexis Sánchez and Henrikh Mkhitaryan in their sides’ respective set-ups brought home how much sense the dual transfer makes.
Mkhitaryan never found his feet under Mourinho but seems to work far better in the Arsenal line-up, while Sánchez’s work-rate was much needed alongside an occasionally lax Pogba.
Alexis Sanchez's defensive map (crosses=tackles; diamonds=interceptions) and ball recoveries map. Note defensive actions made in deep, left-hand areas.
United’s central midfield trio not quite there
At their best, the central midfield trio of Matić, Herrera, and Pogba is superb.
Pogba offers his driving runs and attacking creativity; Herrera has energy of a true box-to-box midfielder and the ability to smother and snaffle attacks; Matić gives a cool head and competence on both sides of the ball.
However, the structure seems to be boom or bust, and all three have major flaws when it stops clicking for them, as happened in the second half.
Pogba’s defending gets lax and gaps appear on his side; Matić is slow and struggles to recover during repeated quick opposition attacks. He’s only really the most defensive player of the three because of his lesser mobility rather than being an outstanding defensive midfielder.
As for Herrera, United can struggle to unlock defences when relying on him in a creative capacity.
They’re so close, but not reliable to be close enough.
Wenger follows Mourinho’s ‘specialist in failure’ lead
José Mourinho aimed the ‘specialist in failure’ barb at Arsène Wenger in 2014, but it was Mourinho who turned failing into a specialism last season.
The Premier League slipped so low on his list of priorities relative to winning the Europa League (as a route to the Champions League) that it practically became tanking.
Today, it was Wenger taking a leaf out of Mourinho’s book.
This was a symbolic game. Not only was it Arsène Wenger’s last match against the old enemy, and his last at Old Trafford, but Arsenal needed to win to stay within mathematical possibility of a top-four finish.
However, Wenger started a heavily rotated side in the hope of beating Atlético Madrid on Thursday and getting through to the Europa League final. It made practical sense, but it still felt a little sad.