Without a doubt, it is the most anticipated game of the first round.
Ruben Amorim will want to get the team playing differently and winningly in the first game. I think this is a decisive game, despite being the first of the official season. It's very important for him that Man United comes out on top, to break that idea that they won't make it.
On the other hand, Arsenal has a team that already knows each other well, has strengthened well, and wants to gain a lot of points from the start.
For the beginning of the season I followed, Arsenal always performed well. Their habit of entering the half of the season changes unlike the beginning of the season.
If the opportunity is more convincing Arsenal than Manchester United, then it is hesitant to be chosen because Manchester United in welcoming this season we can see that Amorim has been filled by new players with good sharpness during strengthening their respective teams in the previous season.
It could be that the Manchester United vs Arsenal match on Round 1 at Old Trafford is difficult for people to make predictions with difficult analysis levels.