Arsenal playing against Manchester United is a good test, though Arsenal has better chances of winning the game because they have always had better performance.
But if Manchester United is able to play tough against Arsenal, and even if they don't win the game, it is a good test to show that Manchester United's performance might be different this season. I just hope Manchester United makes the game interesting.
For Manchester United, this match will be a truly crucial test. The players' motivation will be even higher this season if they even manage to hold Arsenal to a draw. However, if United loses heavily, I'm sure their motivation will waver slightly. Therefore, for United, even a draw in this match would still be considered a significant achievement. Or, even if they don't win, or even lose, if they can put up a strong fight, it will still make them look good and hopefully improve.
But I personally would really favor Arsenal because I think Manchester United will still struggle against a strong team like Arsenal. It's not that I'm underestimating Manchester United, but I simply have more confidence in Arsenal.