Barcelona meets Man United, Barcelona is not doing well in UCL but is doing well in their domestic league (La liga) at least based on the position in the league, it's a different story for Man United (as they are still in transition at the moment) but the result of this match remains depending on how many players in both teams are still fit/injury free after the busy schedule of these few matches.
In history Man United have always struggled against Barcelona, but I think Man United know it has been fixed and are ready. Man United vs Barcelona is by far the biggest Europa league game of all time.
What I have noticed about Barcelona this season is that their performance in laliga is different from other leagues competitions, that is why am not convinced of Barcelona can defeat Manchester United. Before now Manchester United performance in Europa league have been very impressive. Barcelona focus more on laliga and might not put too much seriousness in europa league. It is true Manchester United have always been struggling with Barcelona but a different case might occur.
They be disappointed how quickly they got knocked out of the Champions League so they shift the focus down to La Liga which look like a better competition to them than the Europa league competition.
Camp Nou might be a place of fortress for Barcelona and so is the same for Manchester United who has turned the Old Trafford to a fortress also. If United happens to draw the game at Camp Nou they're ending it all at Old Trafford.