1. Louis van Gaal isn’t afraid to complicate things

It was seemingly not enough for Manchester United to deal with a remarkably extensive injury list - added to 16 minutes in here with Luke Shaw limping off to be replaced by Ashley Young - so manager Louis van Gaal decided to revert to a back three in defence.

It is a system United’s players are palpably uncomfortable with - hence why the Dutchman abandoned the formation not long after the season started - and they were hugely fortunate to get through the first-half unscathed.

Arsenal 1 Manchester United 2 - player ratings Show all 24 1 /24 Arsenal 1 Manchester United 2 - player ratings Arsenal 1 Manchester United 2 - player ratings Wojciech Szczesny Didn’t have to make a save before he was beaten by a delfection of his own defender having clumsily punched away – not in the same class as the likes of De Gea and Courtois. 5/10 Getty Images Arsenal 1 Manchester United 2 - player ratings Calum Chambers Another strong performance from the full-back who at the moment is appearing to be a far better value signing than his injury-prone former Southampton teammate, Luke Shaw – made a vital challenge to foil Rooney in the first half. 7 Getty Images Arsenal 1 Manchester United 2 - player ratings Per Mertesacker Wasn’t asked to do a lot of defending – made some solid headed clearances as you’d expect from him. 5 Getty Images Arsenal 1 Manchester United 2 - player ratings Nacho Monreal Rarely called into action and powerless to keep out Rooney as United overran Arsenal at the end. 5 GETTY IMAGES Arsenal 1 Manchester United 2 - player ratings Kieran Gibbs Despite his own goal he was one of Arsenal’s better performers on the day as he has been for much of the season. 7 Getty Images Arsenal 1 Manchester United 2 - player ratings Mikel Arteta A strong influence on the game as Arsenal dominated the first half but failed to maintain that high level after the break. 6 Getty Images Arsenal 1 Manchester United 2 - player ratings Jack Wilshere – Spurned a great opportunity to give his side the lead and was then fortunate to stay on the pitch after an altercation with Fellaini before going off injured early in the second half – a real mixed bag. 6 Getty Images Arsenal 1 Manchester United 2 - player ratings Aaron Ramsey On his 200th appearance for the club he failed to produce the incredible form we saw from him last season as he was outshone by his teammates. 5 Getty Images Arsenal 1 Manchester United 2 - player ratings Alex Oxlade-Chamberlain He seems to do so many things very well – he was a constant menace to the opposition whether it was his passing or direct running with the ball – the best player on the pitch. 8 Getty Images Arsenal 1 Manchester United 2 - player ratings Alexis Sanchez So often the man to dig Arsenal out of trouble this season but it wasn’t his day – put in his usual effort but couldn’t find the end product on this occasion. 6 Getty Images Arsenal 1 Manchester United 2 - player ratings Danny Welbeck Wasted a good opportunity early on but linked the play extremely well throughout and caused the inexperienced United backline a lot of problems – has to be more ruthless in front of goal. 6 Getty Images Arsenal 1 Manchester United 2 - player ratings From the bench: Olivier Giroud A positive for Arsenal fans to take from a pretty miserable day – the Frenchman marked his return from injury with a thumping strike into De Gea’s top corner. 7 Getty Images Arsenal 1 Manchester United 2 - player ratings David De Gea Kept United in the game throughout and made world-class saves from Welbeck and Wilshere, but will be disappointed to have been beaten at his near post by Giroud. 8/10 Getty Images Arsenal 1 Manchester United 2 - player ratings Chris Smalling As the senior centre-back, much of the responsibility for United’s frailty must rest with him, but he reacted well to deny Welbeck with the score at 1-0. 6 Getty Images Arsenal 1 Manchester United 2 - player ratings Paddy McNair Arsenal’s movement and quick passing exposed the young centre-back repeatedly in the first half. 5 Getty Images Arsenal 1 Manchester United 2 - player ratings Tyler Blackett Failed to cope with Oxlade-Chamberlain’s darting first-half runs, but made a timely clearance in the second half to preserve United’s lead. 6 Getty Images Arsenal 1 Manchester United 2 - player ratings Antonio Valencia Arsenal’s pressure meant he struggled to impose himself on the game from his right wing-back role, but it was his low strike that was deflected in by Gibbs for United’s opener. 6 Getty Images Arsenal 1 Manchester United 2 - player ratings Luke Shaw Limped off with an injury after 15 minutes. 5 Getty Images Arsenal 1 Manchester United 2 - player ratings Marouane Fellaini Apart from his flashpoint with Wilshere made little impact as United struggled to control the midfield. 5 Getty Images Arsenal 1 Manchester United 2 - player ratings Michael Carrick Didn’t provide his usual calming influence on the ball as United conceded the majority of possession to the hosts. 5 Getty Images Arsenal 1 Manchester United 2 - player ratings Angel Di Maria Provided United’s only threat in the first half but faded in the second, although his pass for Rooney’s goal was perfectly weighted. 7 Getty Images Arsenal 1 Manchester United 2 - player ratings Wayne Rooney Worked hard for the team in a deep-lying role without creating much, but took his goal brilliantly. 7 Getty Images Arsenal 1 Manchester United 2 - player ratings Robin van Persie Was utterly anonymous against his former club, and was replaced by James Wilson. 5 Getty Images Arsenal 1 Manchester United 2 - player ratings From the bench: Ashley Young Posed a real threat down the left after coming on for Shaw, particularly in the second half. His cross led to the first goal. 7 Getty Images

Chris Smalling, Paddy McNair and Tyler Blackett flanked by Antonio Valencia and Ashley Young is an almost unbelievable defensive line-up for United in such a big game yet they gradually found their poise after the interval and were able to somehow absorb the pressure, allowing United to counter-attack to telling effect. It looked a mistake from the outset yet Van Gaal and his team found a way to win.

2. The case for Arsenal having a mental block against United grows further

When asked on Friday morning, Gunners manager Arsene Wenger denied his team had any psychological barrier to overcome against United but they have now won just one of their last 15 meetings, losing 11. They were not inhibited in their play - as has often been the case - but lacked conviction in the final third during a first half they dominated and created a series of clear-cut chances.

Danny Welbeck missed a hatful of chances (Getty Images)

Danny Welbeck was among the most prominent culprits - although Wilshere also missed a sitter - and although the £16million man has impressed for England and intermittently for Arsenal, he still has just two Premier League goals to his name. Although Olivier Giroud is now fit after a broken leg - he played the final 13 minutes here - the Frenchman also has questions to answer regarding his efficiency against the top sides despite his marvellous late strike. Wayne Rooney’s clinical finishing for United’s second was an awkward contrast.

3. Jack Wilshere isn’t tall enough to headbutt Marouane Fellaini

This fixture may not have the ferocious intensity of a decade or so ago when Arsenal and United formed a duopoly at the top of the Premier League but the latest instalment was a spiky affair in which Wilshere and Fellaini provided the most controversial flashpoint.

Fellaini won a soft free-kick on the edge of the box and Wilshere, after furiously protesting at referee Mike Dean, then pushed his head into Fellaini’s chest. It was a clear motion towards the United midfielder and he was fortunate not to be sent off. Ironically, Wilshere would then have not been on the field to pick up what appeared a bad looking ankle injury in a tackle with McNair which forced him off 10 minutes into the second half.

Wilshere looked to have head butted Marouane Fellaini (Getty Images)

4. Wenger’s summer spending looks more incomplete with each passing week

Arsenal have kept just three clean sheets in the Premier League all season - against Aston Villa, Sunderland and Burnley. As much as they dominated this encounter, there was always a feeling they would prove vulnerable to a United counter-attack and Valencia’s goal was a calamity from Arsenal’s point of view. Kieran Gibbs clattered goalkeeper Wojciech Szczesny as he attempted to deal with Fellaini before deflecting Valencia’s shot into his own net.

It is an assumption that different personnel would have defended better but Arsenal have made such a habit of conceding bad goals that the decisions not sign a replacement for Thomas Vermaelen or a robust defensive midfielder continue to haunt Wenger. The boos from the home crowd at full-time were directed at the manager as much as anyone - Arsenal continued failings in the big matches do not reflect well on Wenger.

5. Robin van Persie is under pressure to justify his place

Given he spent eight years at Arsenal, Van Persie’s motivation for this game should have been self-evident but this was another subdued display from the Dutchman. With rumours persisting over his long-term future at Old Trafford, Van Persie is under pressure to produce the kind of match-winning performances that won United the title in Ferguson’s final season in charge.