Super League entrants Milan are not yet assured of Champions League qualification and were stunned by Sassuolo at San Siro on Wednesday.
Milan's hopes of Champions League were qualification were hit by a 2-1 home defeat to Sassuolo in their first match since playing a part in European football's remarkable Super League saga.
The Rossoneri were among 12 elite clubs signed up to play in a controversial new continental competition until England's 'big six' backed out on Tuesday and prompted its collapse.
Milan have not played in the Champions League - the tournament the lucrative Super League sought to rival - since 2013-14 and have work to do to clinch the top-four finish that would end that wait, barring any UEFA sanctions.
Substitute Giacomo Raspadori's quickfire late double, after a superb Hakan Calhanoglu opener, leaves the second-placed side still well within reach for Napoli in fifth, six points ahead but with a game in hand to come for the Partenopei.
Milan only confirmed their own withdrawal from the breakaway competition five hours before kick-off and they were then a little slow to get going at San Siro, grateful to goalkeeper Gianluigi Donnarumma when he blocked from Jeremie Boga, who had sold Fikayo Tomori with a dummy.
Calhanoglu supplied the breakthrough strike on the half-hour, though, as he shaped a gorgeous finish into the top-right corner from the left side of the penalty area.