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.
We do not even have to look that further back, Barcelona lost to Frankfurt last season, who ended up winning it all to be fair, but they did and that's the point. It's not about how close these teams are to each other, sometimes close teams end up having a rough matchup, sometimes one wins very easily. Even more important, sometimes bigger teams lose to smaller clubs and gets eliminated.
So, we have absolutely no idea which one of them will win, we have no idea about any of the teams and whoever will be on the next round. We just try to make guesses and so far it looks like we are going to end up seeing Barcelona winning if we make a guess today, but it could definitely change in a month.