Barcelona is a team that is in a more comfortable position compared to Manchester united, they are advancing in the league as they wish, while their closest rivals lose points, they continue to progress by opening up the difference in points. The absence of Dembele does not cause much trouble to Xavi's game, at least there is an alternative that can fill his place. On the one hand, Manchester has to think about the league side, they have a lot of injuries and it will be difficult to replace those players.
When comparing Barcelona and Manchester United you should understand laliga and Premier league are not the same, one league is tougher than the other. When Manchester United and Barcelona meets then I will which team is better than the other. Barcelona is doing very well in laliga by leading the league while Manchester United is in the 3rd position in laliga, this shows the results of this teams in their respective league but since the toughness in these league are not the same. The Manchester United and Barcelona game will decide which of them is better in performance.