The Granata had never in their Serie A history conceded seven goals in a home fixture, while the final whistle blew with some players in tears.
"I have come here to apologise for an indefensible performance. I have already told the players we are going straight into a training retreat, then we'll see what happens," Mazzarri told DAZN.
“Everything was indefensible today. This performance was unjustifiable. All we can do is apologise.
"There is nothing more to say."