Manchester united outplayed Arsenal, the statistics is there. But Arsenal did a great job in the defend, they park the bus and did greatly, the pressure was much on their defenders, the Manchester united attackers were too fast in the attacking line, and if they keep playing like this, they will end in the top four. The only mistake the Amorim made was not to start Benjamin Sesko in this game.
I understand that yesterday game felt like Manchester United did outplayed Arsenal even though they lost in the end but I think otherwise Arsenal were just smart about yesterday's game, they understand the season just begun and all they needed from that game was a clean win without having to stress their players out so they gave Manchester United the chance to do all of the playing and they utilised their only opportunity, this will be clear when Manchester United and Arsenal meets again for the second time, I doubt if Manchester United will still outplay arsenal by then.
Regardless of the goal Arsenal scored which I think was just a lucky one, Manchester United played awesomely, they had more ball possession, they has more short on target and they had the overall better performance than Arsenal in that encounter. So for me I don't think there's any special thing Arsenal played to say the wanted to keep a clean sheets from the beginning of the season, they had to stand their feet to protect the only goal they had till the final whistle which made them fall back a lot of times, this was far from the Arsenal of Last season that will be winning a team and still keep them under pressure.