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Of course Manchester United performance is decrease worse if we look at their final results in the last two matches. I don't expect them to be satisfied with their performance last season in making it back to the Champions League and Erik Ten Hag performance was highly praised last season. But in fact he failed to maintain his best performance at the start of this season after making many blunders in the transfer window this season, such as releasing De Gea.
Yes, it seems like they still want to be in last season where their performance was quite good and they could finally re-enter the UCL. Their enthusiasm was also quite great last season.
However, that has changed quite a bit, even though it's still only the start of the season, but to be honest, Ten Hag didn't start the season well. And his team's motivation also doesn't seem to be good. 3 losses from 5 matches is quite a bad thing for a supposed top team.
Manchester City is a team that tends to put pressure on Arsenal rather than the other way around. I don't think Arsenal can take first place from Manchester City if in the next match they fail to win the match against Tottenham. Of course Arsenal and their fans are very confident in the possibility, but in fact Arsenal failed to have a good mentality in the title race last season.
It cannot be denied that Arsenal is indeed strong and their performance is only getting better. The squad cannot be underestimated, because they are very consistent and solid too. But if we talk about opponents with Man City, it seems that we cannot easily say that Arsenal will beat and take first place from Man City. Because, as @skarais explained, Man City is not a big club whose enthusiasm can easily be reduced, but Man City is a top club that is very solid and consistent, they also have strong players both in the front, middle and back lines. They are not an easy club to beat (although nothing is impossible). but so far, their performance has been the most consistent, with the goals they have created quite a lot so far even though it is only at the start of the season.
Later on October 8, we will see the big match between Arsenal vs Man City at the Emirates Stadium. Of course this will provide a plus for Arsenal. However, it is not guaranteed that they will excel. Because Man City is always ready wherever they compete.