Having a draw in their last match by Manchester united is still a good result, and shouldn't be used to judge their match with Barca. EPL is different and tougher than Laliga,which gives Manchester united more advantage over Barca. It will be a difficult and interesting match,since it has been long since this two clubs clashed.
What's interesting is that the two teams, both Manchester United and Barcelona, are both experiencing an increase in performance in their respective leagues so that their full strength will definitely be seen in the Europa League matches this season and every coach from the two teams will also be very aware of this. they are facing is not a weak team and clearly needs more improvement before the game starts.