I think there's no need to discuss about Barcelona vs Napoli and Arsenal vs FC Porto, it's a sure win for both Barcelona and Arsenal, Napoli and FC Porto aren't that good compared to last season.
C'mon bro, you're not new to football. You can't really believe a sure win is possible in a competition like this. This is the champions league where anything is possible. We've seen giants of the game get knocked out my unexpected teams.
I can partially agree with Arsenal definitely knocking out Porto because Arsenal is in a good form and Porto are really bad currently, but Barcelona and Napoli are on equal form if you look at it more closely. Barcelona have been horrible, probably worst than Napoli who have also been bad. So saying it's a sure win for Barcelona is not true to me. Barcelona has a upper hand, but ist not a sure win.
You must also remember that they matches would be player in February next year, that relatively a long time from now. Teams can loose form or gain from during this period. Players can get injured and coaches can be changed and make the team better within this period.
Barcelona and Arsenal are favorites, but you can't be a hundred percent certain that they'll win.
We all know that when big teams are playing against smaller teams especially in the knockout UEFA Champions League competition, it's always rare to smaller teams eliminate the bigger teams but I think there are exceptions to that popular belief. The Arsenal vs Porto and Barcelona vs Napoli round of 16 ties I don't think are easy games that that can be easily predicted.
Arsenal we know have been impressive since the start of the season but when it comes to the UCL, Porto is a more successful club than Arsenal as they've won the competition before now. So you don't expect a former winner of the competition to be easily beaten. Porto understand how the UEFA Champions League competition is played and would definitely want to make a statement in their tie against Arsenal. Just like bookmakers, I'm backing Arsenal to progress but I expect the two-legged games to be a difficult one for both sides
Barcelona are the reigning champions of Spain and so are their Italian round of 16 opponent Napoli are the reigning champions of Italy and both teams have not been performing like that they did last season in their respective leagues. However, since Napoli sacked their former manager Rudi Garcia and hired the current manager Walter Mazzarri, they've improved their performance both in the league and in their final group stage game and that's why I don't expect Barcelona to easily eliminate Napoli in the round of 16.