The goalkeeper says the loss in Bergamo stunned the squad and that the whole team - including him - made mistakes that cost them in the end
German Denis netted twice past the Liverpool loanee in the second half before Maxi Moralez finished off the Partenopei at the Stadio Atleti Azzurri d'Italia.
Reina, 31, says Rafa Benitez's squad were stunned by the loss and are now keen to make amends for their error-strewn performance in Bergamo by beating Roma in Wednesday's Coppa Italia semi-final first leg.
"We did not expect this defeat," the Spanish shot-stopper told Rai Sport. "We made too many mistakes, many of which were my own.
"I feel responsible, but now we must look ahead to midweek."
Napoli's defeat leaves them six points behind second-placed Roma - who have a game in hand - in Serie A, but Fiorentina's defeat at Cagliari means that they still have a three-point cushion in third.