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.
you're just ranting, every team wants to win in every competitions they are. If Manchester Uinited eliminated Barcelona from the cup, they will feel bad. Because Barcelona are not sure if they will win the Laliga title. They have to fight for every title this season. No doubt the game will be very tough one, because both team are in form right now. I will be must watch game for me. I'll not miss it.