Barcelona will probably not win against Man United in the second leg but it's not because United is the better team. It will be because of Injuries of Barcelona. 3 of their main players are injured or suspended. Dembele, Gavi and Pedri won't be available for the second leg.
United were outplayed by Barcelona in the first leg.
I'd have to disagree with you on the point that Manchester United are a better team than Barcelona. United have played 14 games this year, they've had 1 loss, 3 draws. conceded 11 goals while Barcelona have played 13 games and had 12 wins, 1 draw, conceded 9 goals. They've not lost a game this year.
Not to talk of the fact that Barça are top of their table while United is 3rd. Even without their key Barcelona players the game is still very close so talk more if their players where complete.