AC Milan's season went even further off the rails on Sunday after a humiliating 5-2 home defeat at the hands of Sassuolo left them at risk of dropping out of the Champions League places.

Everything that could go wrong for troubled Milan did at a gloriously sunny San Siro, with two goals ruled out for offside and a shocking defensive display condemning the champions to a second straight heavy loss after being hammered 4-0 at Lazio on Tuesday.

Stefano Pioli's side dropped down to fourth and will find themselves a whopping 15 points off the Scudetto pace if league leaders Napoli beat fierce rivals Roma in Sunday's late match.

Milan's title defence lays in tatters and they will also drop out of Serie A's top four if either Roma or Lazio, who host Fiorentina later in the day, avoid defeat.

Domenico Berardi was the star man for Sassuolo, who are 16th after winning for the first time since late October, the Italian having a hand in Gregoire Defrel and Davide Frattesi's goals which put his team two ahead after just 20 minutes.

Olivier Giroud, who had a fine early finish disallowed, then headed Milan back into the game but Berardi reestablished Sassuolo's two-goal lead with a header of his own from a 30th-minute corner.

Pioli brought on Rafael Leao at half-time after leaving his star attacker out of the starting line-up but Milan were three goals down seconds after the restart, Armand Lauriente smashing home a penalty after being brought down by Davide Calabria.

And an awful afternoon turned into a nightmare when Matheus Henrique drilled in the fifth with 11 minutes remaining, leaving Divock Origi to curl home a beautiful but entirely irrelevant consolation goal for the hosts.

Later Paul Pogba could make his first Juventus appearance since returning from Manchester United last summer after being named among the substitutes for their clash with surprise package Monza.

France midfielder Pogba injured the meniscus in his right knee in July and missed out on the World Cup.

He initially elected not to go under the knife in a bid to make the Qatar tournament but despite later changing his mind he didn't recover in time to play for Les Bleus.

Juventus sit 11th in Serie A after being docked 15 points for illicit transfer activity and will be overtaken by promoted Monza should the Turin giants lose to the club owned by Silvio Berlusconi as they did in September.