1) Man United couldn't live with Arsenal's early tempo
Arsenal are very much about guile rather than brute strength, but they hit Manchester United like a steamroller early on. The Gunners were simply too quick for United's midfield and defence, in both the legs and the mind, as they swarmed forward in numbers and created chances at will. Louis van Gaal's increasingly befuddled players simply couldn't cope with Arsenal's breakneck tempo, initiated by the likes of Santi Cazorla, Mesut Ozil and Alexis Sanchez.
Arsenal had an incredible 96% of the ball in the opening few minutes as they went straight for the jugular, and Ozil's goal followed only 74 seconds after Sanchez's opener.
Partly because they were able to ease off once they took command, Arsenal actually only had 38% possession over the course of the whole 90 minutes and completed just 60 passes in the attacking third. But the passes they did complete were incisive, producing 11 shots - a pretty impressive ratio.
The Gunners also completed 16 take-ons, while United succeeded with only 22 of 38 attempted tackles. Petr Cech or David Ospina in goal? It barely mattered with a performance like this. "We had a convincing performance, a strong united response to our defeat against Olympiakos," said Arsenal boss Arsene Wenger. "We started very strongly, we decided to press very high, to play at a great pace and we did that very well." His counterpart, Louis van Gaal, responded: "We didn't perform to our game plan. We were top of the league so I was amazed. When you give a team like Arsenal so much space to play football then you know that you shall lose. We prepared ourselves to play more compact, but don't lose your aggression. Arsenal were more aggressive than us. We lost in a way you cannot lose when you are top of the league."
2) No thrust in United's attack
Maybe it was the faded shorts. Chelsea have endured nothing but misery since donning the curious black and grey combination that is their new away strip, and this time it was United who were struck by the curse that seems to accompany the garments. United do not trail 3-0 at half time too often, and this match bore echoes of a nightmare first half at The Dell in 1996, when they changed out of their grey shirts at the interval, 3-0 down to Southampton, claiming the shirts somehow made the players invisible.
Whatever the reason, many of them were invisible again at the Emirates Stadium on Sunday, as United conceded three times in the opening 20 minutes of a Premier League match for the very first time. That was down to Arsenal of course, not the shorts. But while the Gunners were nimble in possession, United were laborious, their tempo way too slow as they did precious little with their 62% share of the ball. They completed 547 passes to Arsenal's 314 but much of it was slow and sideways. Of the 631 passes they attempted, only 322 of them were forward, while 309 were either backwards or square.
3) Sanchez destroyed Darmian
Sanchez's second goal definitely came into the 'what a hit, son' category. A brilliant strike from just outside the box summed up a thoroughly dominant performance form the Chilean, who now has six goals in his last three games. Yes, that's as many as Sergio Aguero. Like Aguero, who netted five in one game against Newcastle on Saturday, Sanchez started the season slowly after the exertions of a triumphant Copa America campaign. But he has now clicked into top gear in devastating fashion. Sanchez was at his marauding, energetic best from the very first whistle of this game and caught Matteo Darmian napping to ghost in at the near post and brilliantly backheel home the opening goal.
Darmian also made a poor attempt to stop Sanchez's second, tamely hanging out a leg that was not enough to stop the forward roaming infield to get his shot in. The Italian right-back missed 3 of 5 attempted tackles before being subbed at half-time for the second successive away game, having also struggled against Dusan Tadic at Southampton a fortnight ago. Without Luke Shaw and Marcos Rojo, United now have big problems at full-back. Sanchez also created 2 chances, completed 3 out of 4 take-ons and made 9 ball recoveries during the game. "Very efficient" was how Wenger summed up his performance.
4) Ozil and Walcott key for Arsenal
Ozil hasn't always reached his full potential in an Arsenal shirt, but the ability has always been there and there could be no doubting his display on this occasion. He played a key part in the way United were ripped apart early on. His intelligent passing and movement led to him providing the assist for Sanchez's first goal - delivering a cross from the right flank, where Ashley Young had been asked to play at left-back for United.
Ozil then neatly slotted home the second himself. In total he had 4 shots and was successful with 15 of 17 passes in the attacking third. Walcott was equally dangerous, showing once more what he brings to this Arsenal side when played up front from the start. He may not have scored, having netted 12 times in his previous 13 league starts, but he was a constant threat with his pace and set up the second and third goals.
Asked if it may have been Walcott's best performance as a striker for Arsenal, Wenger said: "In this position, yes. Walcott gave a hugely committed performance. He didn't score but I like to praise the strikers when they don't score but contribute and give assists, and he was involved in two goals. He has shown he can fight and protect the ball as well. Some aspects of his game as a centre-forward are improving."
5) Worrying signs for Schweini
Van Gaal refused to enter into a debate after the game about why he selected Bastian Schweinsteiger to start the match ahead of Morgan Schneiderlin. "I thought in this game I could do it with Schweinsteiger and [Michael] Carrick," the manager said. A World Cup-winner with Germany and veteran of great success with Bayern Munich, there should be little concern about pitching Schweinsteiger into such a match. But it didn't work. Schweinsteiger is perfect for helping United to dominate possession with slow-paced but accurate build-up play. He played a key role in their 45-pass goal against Southampton two weeks ago. However, at 31 he wasn't perfect for dealing with an energetic Arsenal midfield who were determined to seize control early on. By the end he had completed 79 passes, second only to Carrick and comfortably more than the highest tally of any Arsenal player, Santi Cazorla with 49. But crucially Schweinsteiger was a passenger in the early stages and those were the moments that cost United. The German completed only 3 of 7 tackles.