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 .
Barcelona also considers the Europa League to be a very serious league for them, because Barcelona still wants to bring home more trophies in different competitions this season. And today Barcelona has just celebrated its victory in the Super Cup after beating Real Madrid and is clearly still eager to get another trophy in its domestic league such as La Liga and also for the Europa League. Because Barcelona did not take the Europa League lightly for this season.
I agree with you, maybe Barcelona will take their Europa League game as a place to prove that they have stepped up now and are ready to become one of the best teams in Europe again. I mean the previous season they played not as usual and couldn't even compete with other teams in various competitions, so that made them a little underestimated by the team that would face them. The Super Cup trophy means a lot to them to be able to win the next trophy this season.