Reports claimed the Giallorossi's president Dan Friedkin and his son Ryan turned on the players in the dressing room after the loss.
"That's not true," Fonseca said at a press conference on Wednesday.
“I want to say more. Since I've been here I've always had respect for the work of journalists, but this news is a big lie. It's not serious, and I can't accept it."
Ahead of the Europa League tie with Young Boys, the coach continued: “The result is always the most important thing, but the performance counts too. The team must do the usual work, play normally, without focusing on the Napoli game. The analysis is over, that's another match and we play to win.
“First we need to understand what happened, but without drama. We lost a game but the players have understood and remain confident. The result against Napoli does not change anything, the atmosphere remains good and I have no reason to doubt.
“I try to be always balanced when I speak. One thing is the analysis that I do with you, another one is the analysis that I do with my players.
“Everyone understood and I have no problem telling them what I think. But this analysis is reserved for the team."