When we say as a bettor, in this case, looking at the odds that Manchester United now has, with 4.20, it looks good, but this time the Play off at Camp Nou clearly won't go as smoothly as expected.
Regardless of anything, seeing their H2H and playing at home is clearly very profitable for Barcelona for the first leg.
I don't think maximizing a draw at Camp Nou is not a big problem but to make it a win for Manchester United I think it's a bit difficult.
I agree that if you want to bet on them to win with this odds, it's not a terrible bet. Looking at how they have been very well since the return from world cup, I would say they do not have that low of a chance to win, they have a very good chance to win, not the favourites to win the game, but I would put it somewhere at 2.70 to 3.00 levels and not 4.00+ odds, that is "very unlikely to win" type of thing.
In any case, I am going to bet on them to win, but not going to bet too much because there is a big chance Barcelona could win too, they are playing well enough and have a good team and will likely get this game, but their odds are not worth the risk at all to me.