Barcelona's performance in La Liga is not as bad as it is said, you definitely know that Barcelona has only suffered 1 defeat in 10 matches even though they are 4 points away from the current leaders. Xavi has a great chance of taking his team to the round of 16 after getting a relatively weak draw group, but everything cannot be guaranteed because the 3 teams they are playing against cannot be underestimated.
Barcelona has won 3/6 group stage matches, especially the first 3 matches. Bayern Munich, Real Madrid and Manchester City are 3 other teams that also have the same record, but on paper Barcelona is a team that tends to be weaker than the 3 teams above, but Barcelona is still a competitive team.
When we talk about top teams (or those teams that claim this status) we need to look at the results in the “big” games. Barcelona have only had one such game this season so far and they lost it. At the same time, in La Liga they drew three times with obvious outsiders, and Real lost points in games with Atletico and Sevilla, so in my opinion this is not just a 4-point difference, it is a more significant difference in the results. In the Champions League, the whole thing will begin from the playoff stage and much will depend on the result of the draw, maybe serious games will begin only in 1/4.