There are so many big clubs in the european league. Barcelona has a very good statistic against MU caused by it was always winning when it faces MU. The only problem is we didn't know what's gonna be the result. We can't also underestimate MU in this case caused by MU has so many great players. it has also the same chance to beat barcelona. I remember when barcelona against inter in the UCL. inter has not become favourite like MU.
we still can't see the strength of the two teams after the world cup and the transfer market. the two teams may be strengthened by their new players later. but there can also be teams that are disadvantaged such as injured players after the world cup.
but if you look at the current strength, I can still speculate Barca will be more favored. team play, Barca will look more solid on all sides. especially for midfield control.