I don't think so. I understand you were putting MU is a bit weaker compared with barcelona. Barcelona itself was seeing la liga is far more important rather than europa league. Even laporta itself was feeling disappointed after barcelona goes to the europa league.
Manchester United are weaker than Barcelona but if they are weaker, it is a little bit only. Do you think Manchester City are weaker than Barcelona? They are stronger than both Barcelona and Manchester United.
If Manchester United can beat Manchester City in Manchester Derby, they can beat Barcelona in Europa League. We should know results are not come from math but by using this example, I want to say Manchester United have chance to beat Barcelona, just have chance. Beating Barcelona or not, we will only know after their two matches complete.
Im thinking about barcelona will always be playing with the main squad in any of competitions. Dembele was getting injury. Dembele will not be playing against MU.
I think that barcelona will not even thinking to use non main squad for the upcoming match in la liga. It's better for barcelona to keep 5 points gap rather than become even closer with madrid.
Maybe they don't have confidence that they can win with a B-team and they try to use their main squad anytime they can. It is difference between good and world-class clubs in their substitution player resource. Without good substitution players, a club can not be competitive enough in all matches and in all competitions during a very long season.
Considering the current performance, Barcelona is much ahead of Manchester United. Barcelona have secured victory in each of the last few matches. Every player in the team is at their best. So Barcelona have more chances to beat Manchester United.