No one doubts Barcelona potential to get through the group phase as group winners, Barcelona quality is better than the other teams in their group. A number of Barcelona mainstay players injuries have had a big impact on their performance, Xavi had to use young players in several matches because his team was experiencing a player crisis due to being in the treatment room.
Barcelona is not yet fully able to compete with other clubs. The defeat that Barcelona just experienced in El Clasico indicates that Barcelona is still unable to maintain its momentum when it is ahead.
UCL which always presents big matches, will make it difficult for Barcelona when facing clubs such as Bayern Munich and Manchester City. There are still several tasks that Xavi must complete to make Barcelona able to compete with other favorite teams.
Barcelona has won 3 matches, they only need 1 more win to ensure they qualify from the group phase, this is very possible because the quality of the other teams in Group H is far below Barcelona.
I don't think there is any doubt that even though a storm of injuries is currently approaching the Barcelona team, I don't think there will be any problems for them to get through the group phase.
And indeed, in the know out round, I think it will be very difficult for Barcelona when they face the top European teams because so far this season Barcelona's performance has not been optimal, this is not a good season for Barcelona, apart from their defeat in the EL Clasico match, but overall Barcelona's overall performance is not yet comparable to top European teams.