I still think Barcelona has a slight advantage maybe 55-60% to win against Manchester United that either 40-45% to win.
I don't think Barcelona will care if they are eliminated by United, they will surely want to win but their focus is on the La Liga, they are top of the La Liga and they just beat real madrid to win the Supercopa, so if they didn't believe before, now they believe that they can win the La Liga from real madrid, and that is the big trophy they want. If the Europa league would be a distraction to them if they go far in it, then it is better they are eliminated by United early. Manchester United have been playing well in the premier league too, but they are not the favorites for it, but in the La Liga barcelona are favorites.
Yes, Barcelona are the kind of club that can only play in one competition, unlike Real Madrid or Bayern. But I don't think Manchester United are a strong contender for Barcelona at the moment. I think Barcelona can beat them and advance to the next round.