I don't think it's fair to say "even a shakey United team can deal with Arsenal". A much less shaky United team lost to Arsenal at the Emirates earlier in the season and that was when Arsenal were not in the form they are in now.
United still do not look like the team that swept all before them a few months ago, They look beatable and far from as convincing as before. Arsenal are coming into form at the right time of the season and it would be foolish to want to play them now. I see United being only slim favourites before the match.
Of course, United will rise their game. Hopefully Arsenal will too. No one is saying Arsenal will walk all over United but I think it's going to be close.
It's gonna be great