Milan boss Stefano Pioli acknowledged "nothing worked" as his side slumped to their first Serie A away defeat since December 2019 against Spezia on Saturday.
The Rossoneri were dismal at Stadio Alberto Picco and failed to register a single attempt on target in a league game for the first time since August 2019 as goals from Giulio Maggiore and Simone Bastoni sealed a famous 2-0 triumph for the hosts.
It marked the first time Milan had lost a top-flight game to a newly-promoted side since they were defeated by Benevento in April 2018.
The result means Inter – who Milan face next weekend – can leapfrog their rivals at the league summit if they overcome Lazio on Sunday.
Milan have now lost three times in the league this season and Pioli said this was the first reverse that was deserved.
"It will be a valuable lesson for all of us; we realise that if we don't perform at a certain level, we will be embarrassed like this," he told DAZN.
"Spezia won deservedly. This is our first deserved defeat of the season as we did not have enough quality or intensity and our opponents quite rightly took advantage.
"We are not a team capable of really winning ugly. We always try to focus on passing, intensity and tempo, so if we lack those elements, we are not as effective.
"We struggled throughout the game, nothing worked, including my choices and the performances of the players. Obviously, this isn't what Milan are like, so we need to prove that from next week.
"Perhaps we weren't sharp enough to react to the difficulties that Spezia provided. We did expect them to be aggressive and had prepared counter-measures, but they didn't work and we made mistakes, so nothing went right.
"If we don't move the ball around quickly enough, we allow our opponents to get back into shape and become predictable. Nothing worked this evening, on the ball or off it."
Milan face Red Star Belgrade in the first leg of their Europa League round-of-32 tie on Thursday and Pioli is expecting a much improved performance from his side.
"We're well aware of how negative this performance was, just as we're well aware of the difficulty of the games coming up and well aware of what Milan are capable of," he added.
"Anyone who saw us in training this week would not have expected this performance, and yet it happened. It was an off night, we need to get back on track.
"This was a bad performance, it's only right we leave the field with our heads down, but from tomorrow we get back on our feet."
With this shock victory, Spezia have now picked up more Serie A points against sides currently in the top half of the table (13) than those they have earned against teams in the bottom half (11).