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.
Did you watch the Spanish Super Cup final which Barcelona just won. Barcelona played like crazy to win trophies, the struggles of Barcelona players were not in vain, they managed to beat Real Madrid with a landslide score.
Let's compare the Super Cup competition with the Europa League, it's impossible that Barcelona don't care about the Europa League trophy. The Europa League competition is more prestigious, of course Xavi will prioritize the Europa League this season, especially since Barcelona failed in the final last season.
Super was nothing compared with europa league. The fact that if in europa league and barcelona will face so many teams from the different league. Just because it was winning super cup and it doesn't mean barcelona can win europa league easily.
There will always be possibility for the past scenario to happen again when barcelona was losing against frankfurt. Even real madrid could also lose against some team in europa league too.
The result can change anytime. that's it