Comment: Myth of Arsenal being good on the ball exposed again
In training last week Jose Mourinho would undoubtedly have practised counter-attacking to prepare for the game against Arsenal, but what he might have struggled to replicate is the level of stupidity the Gunners displayed to hand them chances.
It's hard to believe Mourinho would instruct one of his centre-backs to play a floated pass across his own defence so that they could practise interceptions, because, against good teams, that sort of stuff should never happen. Unless, in the game itself, the centre-back is Laurent Koscielny.
Mourinho probably didn't ask a different centre-back to pointlessly dribble with the ball for a few seconds so that Jesse Lingard knew how best to close him down, because, at the highest level, good players are meant to cope under a tiny bit of pressure. Unless the player is Shkodran Mustafi.
And if Paul Pogba was dribbling in training last week, Mourinho wouldn't have told an opponent to try and challenge him with a shoulder, because, unless that opponent happens to be driving a bus, it's fairly pointless to try and overpower one of the game's most physically imposing specimens off the ball. But Koscielny tried it anyway.
Those were the three Manchester United goals and while no manager sets his team up so that the opposition will have 33 shots and their goalkeeper will be forced to make 14 saves, Mourinho was relatively safe in the knowledge that, at some stage, Arsenal's players would allow him to look like a tactical genius.
Arsenal's reputation under Arsene Wenger has been built on a notion of free-flowing attacking football, but while some of his players might be able to produce moments of brilliance, it's their ability to make the most brain-dead of decisions which continues to cost them.
There are very few players who would have the awareness to recognise that even though they are five yards from goal, a lay-off backwards would mean an easier chance for a team-mate. Aaron Ramsey did; he set up Alexandre Lacazette and Arsenal were back in the game early in the second-half.
Yet, in a far more straightforward scenario in the first-half, with Arsenal attacking at 2-0 down, Ramsey was the deepest of Arsenal's attacking players and tried a ridiculous dinked pass from just outside the United box. It was intercepted and took seven Arsenal players out of the game.
United didn't score from that particular break, but Ramsey's stunning lack of game awareness, followed by a moment of magic, was essentially Arsenal in microcosm.
At times in the first-half, Alexis Sanchez looked like a child who had just learned a new ball trick as he tried several scooped, looping passes over the top of the United defence and consistently gave away possession, again allowing Mourinho's men the exact platform they were looking for. However, at least there was a magnificent chest pass to throw in to the highlight reel.
Arsenal might point to the statistics of their 562 completed passes, including 263 in the final third, as evidence of their dominance, but there are few teams who claim to be at the highest level that leave themselves so consistently exposed when just one of them goes astray.
Arsenal might produce some stunning moments, but in games against similarly matched opponents the notion that they have a team full of players who are good on the ball is being exposed as a myth with every passing match.
Several times already this season Granit Xhaka has coughed up possession in a routine position for a central midfielder, which cost them goals against Leicester, Liverpool and Stoke City and while every goal is obviously preventable, the ease with which Arsenal allow their opponents to have chances continues to startle.
Sead Kolasinac has the power of a tank, and occasionally the touch of one, while Hector Bellerin looks well on the way to joining Theo Walcott, Jack Wilshere and Alex Oxlade-Chamberlain as a player with enormous promise who is going downhill the longer he is at the club.
Mesut Ozil was exceptional on Saturday, but has very few team-mates he can confidently pass the ball to with the relative certainty of getting it back.
At one stage in the second-half he drove at United's defence, passed to Nacho Monreal, went for the return and should have had the chance to then play a dangerous ball across the United goal. Instead, the next player to touch it was David de Gea, taking a goal kick after Monreal passed the ball straight out of play.
One of Pep Guardiola's basic principles is to throw down the gauntlet to opponents by relentlessly pressuring them and challenging their technique. If it is good enough, they can escape the press and attack from the space available elsewhere on the pitch, but more often than not it isn't and his teams regain possession.
In the 3-1 defeat to City earlier in the season Arsenal were unable to cope with such pressure, but the worrying aspect of Saturday's loss was that many of the 103 passes they failed to complete came under relatively minor scrutiny from players whose brain and feet don't often seem to be compatible.
Typically, Wenger pointed to the difference between the teams in front of goal as the reason for the defeat, but until he realises his team aren't as good on the ball as they think they are, Arsenal will always need to score a few in order to get anything from the majority of games against their direct rivals.
Wenger couldn't have imagined Arsenal having more than 30 shots against a team who are meant to be so defensively solid, but the problem is that Mourinho could easily envision Arsenal doing something to shoot themselves in the foot.
The rivalry between the two managers might be bitter, but until he recognises that his players are writing cheques their ability can't cash, Wenger will continue to be his own worst enemy.