But isn't this an argument in favor of Juventus? Firstly, winning one game against a favorite (assuming United or Arsenal are strong favorites against Juventus, which I'm not sure about) is much easier than winning two games. Secondly, if the main rivals are in another part of the grid, then you obviously have an easier path to the final. And we do not take into account the fact that for United and Arsenal this is the second priority and not the first.
I believe ManU and Arsenal are the best teams here and the favorite team to win Europa League. They are in top 3 of EPL which is the best and most competitive around the world. They are way stable than the other teams here. ManU could manage to concentrate more here than in a match in EPL which isn't the case for gunners, so they might not be ready to sacrifice EPL to win in EL.
Juve is certainly a good team but they aren't that stable in terms of management and players together. The other teams might have a chance to advance but I doubt if they pass to the final game.