Mazzarri may have fielded a strong team against the Czech outfit in the Europa League but he was a helpless bystander as Plzen claimed a 3-0 win at the San Paolo in the first leg of their last 32 tie.
His side are now facing an early exit from Europe but the Serie A title is still an achievable target with leaders Juventus firmly in Napoli's sights.
He told sscnapoli.it: "We have made the right moves a million times this season, but it seems that the problem was psychological, we did not go onto the pitch with the right mentality.
"In football this cannot happen. We must grow and improve our mental skills, because nowadays every opponent is competitive. We always have to play with the right attitude.
"I will talk to my squad and we will exchange ideas but now we have to move on. First of all, we think of doing better in Serie A against Sampdoria, then we will focus on the return match."
Napoli goalkeeper Roberto Colombo is suspended for two games while winger Valon Behrami faces a late fitness test.
Sampdoria boss Delio Rossi will not prowl the touchline at the San Paolo after picking up a two-match ban for directing a vulgar gesture at Roma fans during his side's recent 3-1 victory over the capital club.
Rossi apologised but claimed he was provoked by the home support, telling La Repubblica: "I'm sorry for the gesture at the end of the game.
"I have always given respect and expect respect in return. I will not tolerate insults for any reason."
Midfielder Angelo Palombo will play no part for Samp due to injury but Rossi has an otherwise fully-fit and suspension-free squad at his disposal.
Samp can go four games unbeaten with a positive result in Campania after claiming seven points from three games to reach 11th place in the table.
Recent history is against the Ligurians, though, with Napoli earning 12 points from the last five meetings between the two clubs.