This match is really important for both the teams. Both teams are likely to field their best players. After a long time, Barcelona is going to play against a big team. But since Barcelona beat Real Madrid a few days ago, they have great confidence. I see Barcelona as favorite in this match.
I don't think it's a big or small problem now because it's a good opportunity for them to reduce the margin of loss when they have to be knocked out of the Champions League stage so they need to win in this case.
On the other hand, it is true that even if they have met before, it is a big match, but now Barcelona and Manchester United have a new look so there may be some unexpected things.