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.
If Manchester United win Barcelona I don't think Barcelona will have much worried if they don't perform well in the Europa league. What Barcelona is interesting and desperate to get is to win laliga. If Barcelona lose it is nothing to them .
Both are good and have good history in domestic and UEFA so if Manchester United won surely this could be good for them and Barcelona will try to manage things in positive way for the La Liga trophy because currently their main focus is this which will help them for having better strategy for the future but right now any prediction is surely not going to have any positive impact because we have very long way to go and both will surely try to avoid further injuries or any off the field issues because these both are having new units which need more time for development and giving better results for the end of this season.
But right now, Barcelona win against Real Madrid in Super Copa is also announcement about their desire about trophies and having good dominance in La Liga which was under threat for the last few months with their poor results now Xavi brining back things which were important for having winning combination.