Maybe you mean playing badly in the Champions League, not in the Europa League, because Barcelona, who are already in the Europa League now, is due to not qualifying for the Champions League group phase, so they have to be in the Europa League play-off round. And yes Barcelona will try to improve everything in the Europa League as well as in the domestic league as well as has been seen from the results in several matches there (La Liga).
Barcelona is definitely not the first seed in the Champions League group stage this season, that's because Bayern Munich and Inter are very strong teams for a large number of Barcelona players. This failure only made me admit that Barcelona had not been able to compete with other top European teams, but slowly Barcelona proved its capacity.
I can expect barcelona to qualify for the last 16 of the Europa League this season. Manchester United do have a chance, but it is because Barcelona's form is getting stronger that I hope victory and qualifying for the round of 16 is possible for them. It's up to people to take Barcelona for granted, it won't change my view of Barcelona's right now.