I think Napoli without Spalletti will not be able to go further in the Champions League this season. Napoli faced a big obstacle in the round of 16 because Barcelona is not a weak team even though they have not been in good form recently, but in the Champions League, Barcelona performance is certainly much better compared to the domestic league. This match will be a life and death match for Napoli in the first leg because if they fail to get a win at home then they have no hope of beating Barcelona at Camp Nou.
Barcelona wants to record a new history under Xavi coaching so they will try their best to beat Napoli in the round of 16. Napoli don't even have any hope of winning the title this season because in Serie A they are far behind Inter Milan and also in the Champions League they have quite a big obstacle in the round of 16.
Since Spalleti left the club, Napoli has lost its enthusiasm unlike last season, which had high self confidence so that it was able to appear impressive and consistent in the Italian League. After changing coaches, Napoli increasingly showed instability even though they were able to qualify for the next round in the round of 16 Champions League.
In the last 16, Napoli is starting to have bigger problem facing big club like Barcelona. If you compare the performance of Napoli and Barcelona, it is clear that Barcelona is still the favorite, even though in La Liga, Barcelona is not very consistent, but it is very likely that in the Champions League, Barcelona will fight as hard as they can beat Napoli.
Even so, we shouldn't have too high expectations, in fact, sometime team that we doubt can usually appear surprising. Napoli cannot be underestimate because basically the two team's are not very stable, but yes, the competition in the domestic league and Champions League is very clearly different from the others clubs competing in the Champions League will continue to show its best performance there.