The performances of both Barcelona and Manchester United have been erratic. Barcelona won against Atletico Madrid though. But they drew the match against Espanyol in the previous match. On the other hand, the situation of Manchester United is also the same. There is no consistency in their performance either. And so it is difficult to say which team will win the Manchester United vs Barcelona match. Both teams have equal chances of winning. But the first match is Barcelona's home match. In the home match, I will put the Barcelona team ahead. Barcelona will probably win the first leg. But the goal difference will not be too much.
Barcelona and Manchester United have good performance in domestic league and consistency at the winning trend except Barcelona held draw by Espanyol at first match after FIFA World Cup 2022. Current performance from Manchester United have quite well and win all matches after World Cup.
I don't have ideas about which one possibility qualifier to next round in UEFA European League, however between Barcelona or Manchester United have eliminated at playoff round. Nowadays away goals nit important thing after UEFA removed all rule about advantage of away goals.