The Partenopei coach admitted that his side started slowly at the Giuseppe Meazza and he was ultimately disappointed that they did not make the most of their numerical advantage
The Partenopei fell behind on the half-hour mark to a goal from Mathieu Flamini but hauled themselves level less than three minutes later through Goran Pandev.
Flamini was then dismissed during the second half, but Napoli were unable to force a late winner which would have ended Milan's hopes of pipping their visitors to second place in Serie A, and the automatic Champions League group stage berth that comes with it.
"I am satisfied with my team's performance, but there is regret at not having won," Mazzarri admitted to Mediaset Premium. "We could have taken home all three points if we had been a bit more clinical.
"However, our evening has been a positive one. I alway look for a performance and that was there tonight.
"We started with the handbrake still on, as so often happens, but we deserved more from the game after it went 1-0."
Marek Hamsik, who created Pandev's equaliser, was surprisingly withdrawn midway through the second half at San Siro but Mazzarri explained that he felt he had no other option but to replace his trequartista.
"Marek was having cramps, so I didn't want to risk an injury," he revealed.
Napoli's draw at San Siro leaves them eight points adrift of Juventus having played one game more than the leaders, who face Lazio on Monday night.
However, while Mazzarri does not believe Antonio Conte's men can be caught, he is not giving up on the Scudetto just yet.
"Anything can still happen," he pointed out. "On the final day, we will see where we are.
"This year we have collected 15 points more than last year; Juve have six points more than last season.
"But now is not the right moment to evaluate what we are lacking compared to the Bianconeri. We will evaluate everything at the end of the season."