I agree with you. Walter Mazzarri tries to correct Garcia's mistakes. But we don't know yet whether he will succeed. And so far Walter Mazzarri has not even been able to stabilize the Napoli team. Defeat in 4 out of 7 matches is not a good result. And against Barcelona, Walter Mazzarri will solve all the problems of the Napoli team, it seems less likely. For these reasons, I cannot consider the Napoli team as a favorite in the match against Barcelona. Walter Mazzarri needs more time to strengthen their squad. At the same time, Walter Mazzarri will be looking to strengthen the squad by adding players of his choice to the squad. It is definitely time consuming.
Indeed Mazzari has not been able to bring very good changes to Napoli, in fact, the last time they lost quite badly to Frosinone which made them have to be eliminated from the Coppa Italia even though that happened because Mazzari only played more of Napoli's second-tier players but the defeat was quite disappointing.
Seeing how Barcelona is currently, I think both teams have the same chance of qualifying for the quarter-finals and the home match will be quite crucial for both teams.
Whichever team can maximize the home match by winning it with a convincing score, they will have a better chance of qualifying, and because Napoli is the home team in this first leg, so they must be able to maximize this well because the pressure will be greater on Barcelona in the second leg, who playing at home can sometimes be more difficult when they are behind.