The match between Barcelona and MU is still a long time, about 1 month later, with that space, anything change can happen with the squad or team. As I remember several years of situations when a team is favored to win like Barcelona which has a perfect squad has loss meet the underdog team. We can say Barcelona is a team that can not have a problem beating MU, but if 1 of the player have a problem like an injured or can't play on that match, that possible MU will take the opportunity to win and go to the next round.
If Manchester United can beat Manchester City, they can beat Barcelona. Compares Barcelona with Manchester City is unfair too because Barcelona failed in Champions League while Manchester City are one of biggest candidates for a trophy in this season.
In Manchester United, they have some star players who can decide matches so even they are a little bit weaker than Barcelona, they have not too small chance to win.
I agree with you. There are many more days left for the match between Barcelona and Manchester United. And over this long period of time the performance of teams can change. Manchester United's performance was quite stable. But in the last match they could not win against Crystal Palace. So we are not sure if Manchester United can maintain their consistency. And so I would say that Barcelona vs Manchester United match should not be predicted right now.
Many days till their first-leg match and many things can be changed till that day. I really believe they will contribute exciting matches from all good performances they have been showing recently. Europa League is a biggest trophy two clubs can win in this season especially for Manchester United as they are impossible to win Premier League.
With Barcelona, they have another chance to win La Liga but Europa League is an attractive goal for them in this season.