This match will start in the next 5 days but most people have predicted including betting markets that have displayed the odds for each team, just like your prediction the bookies also favor Arsenal 1.80 if we bet on Arsenal and 4.1 for Manchester United the difference in odds is obvious here but still market predictions do not guarantee they are always right, we still remember the match a few days ago when Newcastle were the favorites to beat Liverpool but what happened was the opposite.
A separate outcome does not say anything about the correctness of the quotes, because even if Newcastle won, there would be an obvious discrepancy with the quotes of the bookmakers: we would have 100% of Newcastle's wins, while the bookmakers predicted only 40%
As for the upcoming game, I think that Arsenal is even a little underestimated - they started modestly, but United are just terrible.
I completely agree with you though, but to be honest, which the odds am seeing, I can't say for sure that Arsenal is underestimated as you said, this is particularly based on how Arsenal has been playing lately, they started well indeed, but recently, winning seems to have become a big problem for them, look at your last game with Fulham, the almost lost that game, but at the end of the day , it ended in a draw, something I am really not proud of as an Arsenal fan.
So honestly, I can't see that Arsenal is underrated, in fact to be honest, I think Man United are the ones that are underrated, but all the same, my fingers are crossed and I am really looking forward to watching Arsenal beat Man United, but then again, I am not going to underrate Man United because I believe they do have a chance against Arsenal.