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Xavi Hernandez would encourage his new young players to suit up for the main squad, because it's time to elevate the team to the next level, no longer counting on the primary players, but rather sticking to future plans, which are the young players who are eager to start for the team, which might crumble currently, but they will definitely become stronger in games. I doubt Barcelona will reach a vital stage in the UEFA Champions League this season because they are woefully underperforming in La Liga but are yet to show any signs in UCL competition.
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.