I would say Napoli should now focus only on Serie A matches. Their performances in Serie A have been poor. They have lost 3 of their last 5 matches. Even fourth or fifth in the table is now much more difficult for Napoli. Fiorentina, Atalanta and Roma teams are trying to climb the table. And so if the performance of Napoli is not good, we will see the Napoli team in the seventh or eighth place of the table in the future.
The defeat to Frosione 0 - 4 and it happened at home, was a very embarrassing result for Napoli. They don't have any other schedule, in the UCL maybe they can't do much. Indeed, Napoli should focus on playing in Serie A, they need to improve their ranking in the standings to be able to play in the UCL next season. But, is it possible that Mazzari will be able to change Napoli's condition in the second half of the season? I'm a bit pessimistic. Mazzari needs to change the mentality of the players who are currently down because the match results do not match the expectations. This may takes time and it's unlikely to be enough with half a season left.