Naples has been preparing the party for months: streets festooned with banners as blue as the sky above the Tyrrhenian Sea. Even in Italy’s most superstitious city, they cannot pretend to believe that another team will pip them to the Serie A title. Coming out of this season’s final international break, Napoli were 19 points clear of second place with 11 games left to play. Day by day, more decorations are added: a fresh mural of a tricolore badge celebrating this third Scudetto or cardboard cutouts of players in the Spanish Quarter. Across the city, locals will tell you they are only just getting started. Yet there was no party atmosphere inside their own stadium as Napoli welcomed Milan on Sunday night. Even before kickoff, the atmosphere felt venomous. Ultra groups had declared themselves on strike, protesting against the restrictions on flags, flares and instruments allowed into the stadium, as well as high ticket prices for the Champions League quarter-final against these same opponents. Some chanted insults against Napoli’s owner, Aurelio De Laurentiis. Other fans rebelled against them and sought to drown them out with whistles. Soon, there were fights on the Curva. All this was just a backdrop to one of the most astonishing matches of the season, the champions-elect clobbered by the team whose title they intend on taking. Milan were not even in Serie A’s top four when the game kicked off, butthey won 4-0 at the Stadio Diego Armando Maradona. The rout began in the 17th minute, Brahim Díaz bamboozling two opponents on the right wing before releasing Rafael Leão through the middle for a chipped finish. The Spaniard grabbed the second, bringing down Ismaël Bennacer’s cross and selling a dummy to Mário Rui before sidefooting home with his left foot. Napoli, who have swaggered through this season, scoring 64 goals in Serie A and another 25 in eight Champions League matches, offered no reaction. Olivier Giroud narrowly missed a chance to make it 3-0 before Leão seized his, turning Amir Rrahmani inside out and lasering the ball into the corner. Alexis Saelemaekers, on as a second-half substitute, completed the scoring after weaving straight through the middle of the defence. How does a night like this happen? Napoli had previously conceded 16 times all season. Milan had not won their previous four matches in all competitions. Their last game before the international break was a 3-1 defeat to midtable Udinese. This was a rediscovery of the qualities that made them champions last season, a hungry young side playing searingly direct football with forwards willing and able to beat their man one on one. Brahim was unplayable up to his second half withdrawal. Leão had not scored in his past 11 club appearances, but was utterly ruthless. A good piece of the credit must go to Stefano Pioli. He has switched his tactical approach already this year, moving to a back three in a successful bid to interrupt a losing run back in February. On Sunday, he unexpectedly reverted to the 4-2-3-1 he used through most of Milan’s run to the title. For Brahim and Leão, that meant a return to wider positions, without a wing-back overlapping outside them. “I feel more comfortable playing in this [formation], staying left, waiting for the ball and making the movement inside,” said Leão . “Sometimes when I’m in the centre I think I don’t have space to turn around and do my game. I prefer to play in this space.” The change also allowed Pioli to push Bennacer into a more advanced position through the middle, assigning him to harry Stanislav Lobotka and disrupt Napoli’s build-up play. Milan won at Napoli last season, too, with a tweaked formation that gave Bennacer and Franck Kessie licence to press high and break up attacks before they were started. Does Pioli have Luciano Spalletti’s number? That would be an assumption too far. Napoli won the reverse fixture at San Siro this season, just as they did last term. It is a curious detail that the home team has failed to win the past nine meetings between these teams. The real question is what psychological effect this will have when they meet again for the first leg of their Champions League quarter-final in nine days’ time. Napoli hope they will have Victor Osimhen back for that after he missed Sunday with a leg injury. But you cannot explain away a 4-0 defeat with the absence of one centre-forward, even if he does happen to be Serie A’s top scorer. On Sky Sport, the journalist Fabio Caressa advanced a fanciful theory that Napoli were merely the hustler in a pool hall, lulling Milan into a false sense of security. The league is effectively won, so why not go easy in this game? More realistic might be to say that Napoli became a little complacent. As brilliant as they have been for most of this season, they did lose at home to Lazio last month. The lack of any serious title rival may have led them to take too many things for granted. A toxic atmosphere in the stands will not have helped anyone’s focus either. Spalletti did his best to set a tone, selling a message that his team had no reason to fret. “Since I’ve been here we’ve usually always reacted,” he said. “Sometimes we’ve lost a game, but then in the next one we’ve always responded.” Pioli was likewise reluctant to be drawn into any conversations about how this result would affect their next meeting. Milan finished the weekend in third, but the competition to play in next season’s Champions League remains fierce. “No euphoria,” he said. “This was a good performance against a big team, but we need to think about the next game now.” Even the most stubborn mind will struggle not to skip ahead just a little. Before the game, the former Milan defender Alessandro Costacurta had characterised this first meeting as an uneven matchup, saying: “Napoli are marvellous, in good health, a beautiful group. Milan are struggling and see that Champions League spot for next season getting tougher. That creates anxiety.” They did not look so worried as they thrashed the best team in Italy. Naples will almost certainly get its Scudetto party but before their first Champions League quarter-final, has a small seed of doubt been sown?