Napoli were beaten 1-0 via Fede Gatti's Juve winner.
Mazzarri said: “Anyone who knows football realises there are moments. The lads played a great game, I think they dominated the match, forced Juventus to defend in their own area for almost the entire second half.
“Our only fault was not covering that cross and we lost the man in the box. Napoli tried everything, these things happen.
“I am convinced that if we play like this, we will have a run of positive results and really climb the table."
Mazzarri also said: “Naturally, I just arrived, so I cannot talk about what happened before. However, if they called me in, it means there were already a few problems here.
“I have seen a team that always played good football, better than the opposition, but we don't seem to be able to score goals. If we do not score that Kvara chance, it is a clear sign we are in a negative period. Bad luck happens too.
“I believe if we continue like this, with important players rediscovering full fitness and sharpness, we will certainly get results.
“We currently lack that determination that can be the difference in a very balanced game like this. Juventus are great counter-attack specialists, but we allowed them very little in that way. Some elements need to improve, including the fitness levels of players coming off injuries."