Pietro Iemmello scored the only goal of the contest and the closest Milan came to equalising was when Franck Kessie struck the woodwork with a thunderous effort, and Gattuso admitted his side are still struggling after recent poor results.
Gattuso told Mediaset Premium, as quoted by Football Italia: “It’s clear to see the team is on a downward spiral. When you play without soul, you get embarrassed like this.
“I am the first man responsible, but when we analyse the game later, the alibis will fall apart. When you play against a team like Benevento, who do play good football even if they are bottom of the table, we can’t play with no determination or soul.
“It doesn’t matter if we play with two, three or four strikers, because we don’t have the psychological strength or the vehemence to do anything. We basically tickle the opposition, we don’t hurt them."
Gattuso also apologised to Milan's supporters who watched as the Rossoneri took their winless streak to six Serie A games, with their last triumph coming in the 3-2 success against Chievo.
He added: “We have to apologise to the fans, because we wear a prestigious jersey and we can’t have embarrassing nights like this.
“You could tell we just interpreted the moves completely wrong. Cutrone went wide, because we couldn’t find spaces. During the week there are good signals in training, but then the moment the first obstacle arrives in a match situation, we can’t react. We are empty.
“I can’t come here and say things that aren’t true. It’s obvious we are struggling. We don’t even need the opponents to do anything extraordinary to beat us."