My personal opinion is that Manchester United is just not stable enough as a team right now. They have brought in some new players who are yet to get set with the team. I know that arsenal has also brought in some players. But Manchester United did definitely gamble with the transfer that they had made. And of course the homeground advantage is going to be with Arsenal.
But even though I am saying that Arsenal is going to win the match, Arsenal should definitely not underestimate Manchester United. Which I don't think arsenal is going to do anyway, but that is one reason why Arsenal might lose. They cannot take things easy at all against Manchester United, who has definitely been showing some good performance recently. Manchester United does look to be improving.
If you say that Man United is gambling by buying players, then it's not correct to say it's stable, right? because if it is stable then the choice must be right and not arbitrary. In my opinion, so far Man United is clearly still far from stable if you look at their performance at the start of the season, they stumbled when they lost to Tottenham, where Tottenham should have been even more unstable after losing their mainstay striker. Man United still has the same striker and even added that Mason Mount should be better than last season. In fact not, now they have lost Varane and Luke Shaw due to injuries which are needed to withstand Arsenal's attacks next week. In terms of squad depth, Arsenal and Man United are now much different, the advantage remains with The Gunner, who will make a big loss if Man United loses for the second time.