Arsene Wenger's 60th and final game against Manchester United as Arsenal manager ended in defeat as the Red Devils took all three points right at the death. The final scoreline read 2-1 with the winner coming in the 91st minute.
Paul Pogba gave United the lead in the 16th minute before Henrikh Mkhitaryan grabbed an equaliser against his former team in the second half. However, it was a Marouane Fellaini header in stoppage time that gave United the win to break Wenger's heart.
Here are the major talking points from the game:
#1 Men vs Boys, but the kids are alright
The news spread like wildfire the previous night and most fears were confirmed when the squad that travelled to Manchester did not include Laurent Koscielny, Shkodran Mustafi, Aaron Ramsey, Mesut Ozil, and Alexandre Lacazette.
The average age of the Gunners starting XI - 24 years 67 days - was their youngest since August 2011 when they lost 8-2 at the same venue.
It was a stark contrast to the United side who were not only far more experienced but looked bigger, meaner, and more powerful on the pitch. The likes of young Reiss Nelson were easily brushed off the ball.
Their inexperience also showed when they didn't even bother appealing for fouls which would normally see players up in arms.
The kids may be in the big leagues now, but they have a lot to learn about putting pressure on the referee and not let things slide - especially when Maitland-Niles had a good shout for a penalty but nobody said a thing.
But Arsenal can be proud of this performance despite the loss. They didn't roll over like they did in the 8-2 game and competed with United all across the pitch over 90 minutes.
#2 Arsenal only looked to play on the counter-attack
Despite such an inexperienced starting lineup, Arsenal played with a disciplined approach at the back. They were pretty sturdy with two banks playing a 4-5-1 off the ball as Pierre-Emerick Aubameyang played up top as a lone striker.
Their only attacks came through the pace of Hector Bellerin on the right flank and Reiss Nelson on the left. Henrikh Mkhitaryan made a few good runs through the middle to get involved but he was dealt with quite easily by Paul Pogba and Nemanja Matic in the first half.
The Gunners' main outlet was out wide but Hector Bellerin's crosses were not the best while Nelson was never in synch with Aubameyang's movements. On the rare occasions that they did get opportunities on goal, David De Gea had them very well covered.
However, the Spanish shot-stopper could do nothing about Henrikh Mkhitaryan's goal which came against the run of play after Arsenal had won the ball back in midfield and United had lost their shape.
#3 Manchester United do well to isolate Arsenal players
Although Arsenal managed to bring their passing game to Old Trafford today, there were occasions when the Gunners simply failed to find a teammate.
Usually, that is a result of Arsenal players' lack of movement to get into space and get free. But that wasn't the case today.
Manchester United's players from front to back worked hard off the ball to close passing lanes and isolate Arsenal players when they were on the ball. The Red Devils also used the touchline well to isolate them and close in to win the ball back.
However, if the fans and Mourinho can have one complaint, it was that United dropped that intensity in the second half. Seeing that Arsenal were slowly on the rise after their goal, Mourinho quickly made substitutions to tilt the game in the favour of the home side.
And it worked.
#4 Arsenal youngsters Maitland-Niles and Mavropanos shine
Ainsley Maitland-Niles has played 25 times for Arsenal in four different positions this season. He's only 20 years old.
The young Englishman was deployed in central midfield today alongside Granit Xhaka in a double-pivot and he excelled in that role in the absence of the rested Aaron Ramsey.
Maitland-Niles was at the centre of everything Arsenal did when they had the ball. He carried the ball forward and his link-up play between defence and attack was very crisp.
Behind him, 20-year-old Konstantinos Mavropanos made his debut at centre-back and had a debut to remember. Dinos as he is known, had a 100% tackle success rate, made 4 clearances and 3 interceptions. He even managed to complete a dribble as he advanced the ball.
The Greek defender managed to keep Romelu Lukaku at bay, dispossessed Alexis Sanchez when he had the ball, and then competed with Marouane Fellaini in the air when United lumped the ball to him.
All in all, a memorable debut for a player who has a good future ahead of him if he keeps this up.
#5 United seal Champions League qualification in 'Fergie Time'
Mourinho's substitutions to bring United back into the game saw Marouane Fellaini and Anthony Martial come on for Ander Herrera and Jesse Lingard. Both players had their instructions and they executed them to the letter.
While Martial was tasked with carrying the ball forward and giving them an impetus in attack, Fellaini had just one role; get busy in the Arsenal box. Cross after cross was directed at the head of the big Belgian and, to Arsenal's credit, they kept him at bay for 27 minutes.
But stoppage time saw Fellaini lose his marker (Xhaka), manage to make some contact with a delightful Ashley Young cross, and score with his back to goal as the ball sailed past David Ospina.
It was a dagger to Wenger's heart as well as Arsenal who are yet to win a league game away in 2018. What's even more worrying is that they have only won three away games this season.
On the other hand, United have now mathematically sealed a top-four spot with fifth-placed Chelsea too far behind. One more win will also seal second place for Mourinho's side and keep Liverpool at arm's length.