The Milan club found themselves on the end of a one-sided scoreline at San Siro as Albin Ekdal netted a hat-trick in a sensational away performance from Zdenek Zeman's men, who in turn claimed their first Serie A win of the season.
Yuto Nagatomo's dismissal and another indecisive showing from close-season arrival Nemanja Vidic compounded Inter's woes.
And although Mazzarri acknowledged the impact of the red card, the Italian conceded that he made selection error.
He told Sky Sport Italia: "When a coach loses 4-1 and then comments on a red card, then he gets criticised.
"Of course there are moments in the game that can change it and if we hadn't gone down to 10 then we wouldn't have conceded four, but never mind.
"Nagatomo unfortunately made a mistake on the first goal, so he was irritable and in an attempt to make up for it perhaps went into the challenges with too much emphasis.
"I probably made some mistakes when evaluating the squad rotation.
"I thought the team was in great shape this week and didn't pay enough attention to the fitness levels, so kept some players who were on their third or fourth games in a row. It showed.
"Of course, once it rains then it pours, as we went down to 10 men. An experienced team would've sat back and controlled the situation, waited for the second half and not conceded four in the first half.
"It became impossible to turn it around by that point. It is a lesson, the players will learn from this and I already said that to the players."
