Manchester United did not perform badly in the warm-up match against Manchester City, it cannot be said how Manchester United will perform in the Europa League. There is a lot of difference between a warm-up match and a main stage match.
However, we have high expectations for Manchester United in this season of the European League. If this team performs as planned, they have the potential to do well.
Perform well in a single match and perform well through a long season is very different and with latest seasons, we have enough evidence that Manchester United failed to maintain their performances well throughout a long season. They had some great weeks or months but at the end of season, they failed to achieve their target at beginning of season.
I don't even think that Manchester United even have any targets for a very long time now and the F.A cup they won last season was just a clear coincidence because they didn't even bargain for it so it came as a very big surprise to the club even to their management because the performance of Manchester United for some seasons now is nothing to write home about.
Europa League has many matches, not only one match and Manchester United can be affected by their matches in Premier League too. They can not improve and only play like previous seasons, they actually lack of ability to win Europa League even Bayer Leverkusen will no longer be there.
Actually the Europa league will be a very tough one for Manchester United but considering the fact that they didn't expect to get to the Europa league if not for winning the F.A cup so they would want to make some positive impact even if they may not reach the finals. But they can still take everyone by surprise after all they won the Europa league in 2017 under Jose Mourinho so anything can be expected from the Red devils even if they lack the quality to win the Europa league.