Juventus were handed their second defeat in the league this season and also missed the chance to move six points clear of Inter Milan.
Cristiano Ronaldo netted his eighth goal in eight games in a row, but it was not enough to help Juve after Piotr Zielinski and Lorenzo Insigne put Napoli 2-0 ahead.
Sarri told Sky Sports Italia that he was glad to see his former club, Napoli arrest their slump, even against his current club.
“I am happy for the lads, as I will always be fond of them.
“If you have to lose, I guess I’d rather it be here to help Napoli get out of their current problems. I’d rather they started winning next week, of course.
“Clearly, the game takes over and you have to detach yourself from everything around that. Napoli represented a special moment in my life and it’s always pleasing and emotional to be back here.
“We should have felt more motivated to break away [from Inter] rather than less, otherwise this will mean we are weak. I would hope not.
“If that situation weighed on us, even unconsciously, it means we are not ready yet.”
