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Lautaro Martinez's first-half goal condemned Milan to another chastening defeat at San Siro, meaning it has gone five league games without a win for the first time since 2018.
Last season's Scudetto winners have now lost four consecutive games in all competitions, and it sits 18 points adrift of runaway leader Napoli after slumping to sixth place in Serie A.
WATCH: Martinez adds to Milan's misery as Inter wins derby
Milan failed to attempt a single shot on target in the game after opting to match Inter's 3-5-2 system, and Sacchi says the Rossoneri no longer resemble a coherent side.
"Milan were no longer a team, especially in the first half. Inter did not play against an opponent, but against a sparring partner," he said.
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"Switching to a three-man defence created confusion and erased the last few years of work. Furthermore, any idea of the game was cancelled.
"How many shots on goal did Milan make in the first half? Zero. It was a monologue from Inter who didn't struggle too much and ended up dominating.
"Simone Inzaghi's players are almost all of a high level and all have great experience. You can only win against Inter if you play as a team, something that Milan did not do."
Milan required 58 minutes to attempt a shot in Sunday's game, the latest point at which they have managed their first effort of a Serie A fixture on record (since the 2004-05 season), and mustered just four efforts, with none on target, for an accumulative expected goals (xG) of 0.3.
It faces Torino in their next league fixture on Saturday (AEDT), before hosting Tottenham in the first leg of its Champions League last-16 tie four days later.
