The Giallorossi were tipped for the title in pre-season, but currently lie second from bottom with just one win from six games in Serie A.
"After two consecutives victories we hoped to repeat ourselves in Naples," Riise wrote on his official blog.
"After a good first half, Napoli played better and profited, scoring two goals. We are as disappointed as the fans.
"Now we must work hard and get the three points on show in the next game when Genoa arrive at the Olimpico.
"We are going through the same dark period at the beginning of the season as last year when we changed face and fought for the Scudetto until the end.
"We need to find that spirit again and I am convinced that Ranieri will manage to help us once more in this situation."