Barcelona wasn't in their good form, but it's a big joke if they're lose against Shakhtar Donetsk and Royal Antwerp because both of them are in different level. If Manchester United was in this group, I believe Manchester United will able to qualify for knockout stage.
It crossed my mind, maybe if Man United were in this group then they would qualify for the round of 16. It must be admitted that Barcelona has been very disappointing recently with its performance.
Either Barcelona or Napoli win the match, their next match is against Arsenal which I think they would lose.
I look at Barcelona's recent performance, it feels like any team can provide good competition. However, both teams have almost two months to play the first leg. I think both teams can prepare their respective strengths until then.
I also think Napoli is a blessing in disguise. Because Barcelona plays best when they face their opponents seriously. Maybe Barcelona's current preparation is a finishing problem that needs to be fixed to make it better. If Barcelona can maintain overall team fitness and be more ruthless in front of goal, Barcelona can perform well in the UCL.