"We came back against Milan at the San Siro and we held on to the result... Today there were a lot of positives."
Milan, who finished with 10 men after Romagnoli completed a miserable afternoon by being shown a straight red card for taking out Gregoire Defrel, are level on 32 points with league leaders Napoli who host Lazio in Sunday's late match.
FIFA president Gianni Infantino was among those present in Naples as a statue of club legend Diego Maradona was unveiled ahead of kick-off to honour the one-year anniversary of his death.
Their first home league defeat of the season after last weekend's 4-3 loss at Fiorentina mean that city rivals Inter Milan are now only a point behind in third, with Atalanta a further three back after beating Juventus in Turin on Sunday.
"We were lacking lucidity because we made mistakes in far too many situations," said Milan coach Stefano Pioli.
"We needed to manage our lead better... we made a mistake for which we paid handsomely, and it's clear if you concede seven goals in two games something's not working."
Pioli, who on Friday signed an extension to his Milan contract until June 2023 with an option for another year, made a raft of changes to the side that earned a superb 1-0 Champions League win at Atletico Madrid on Wednesday.
Injuries to Olivier Giroud and Ante Rebic led to Zlatan Ibrahimovic playing up front with a strong support line of Rafael Leao, Brahim Diaz and Alexis Saelemaekers, with Tiemoue Bakayoko making his first start of the season in midfield alongside Ismael Bennacer.
And Bakayoko was at fault for Sassuolo's leveller which came just three minutes after Milan had taken the lead, lazily giving the ball away to Giacomo Raspadori who then fed Scamacca the chance to smash a long-distance effort in off the bar.
Scamacca was also at the heart of the goal which put Sassuolo in front in the 33rd minute, belting a close-range effort which returning Milan goalkeeper Mike Maignan could only push off Kjaer and into his own net.
And Berardi put the icing on the cake 20 minutes into the second half by making a fool of Romagnoli, putting the Italy defender on his face with some nifty dribbling, before calmly finishing the third goal.
Roma are three points behind Atalanta in fifth after Tammy Abraham gave them a 1-0 win over 13th-placed Torino.
England forward Abraham rolled home the winner in the 32nd minute and shortly after thought he would have a chance to net his fifth goal in four games in all competitions from the penalty spot.
However, after waiting over five minutes to take the spot-kick, the decision was overturned for the 24-year-old's heel being offside in the build-up.
Bologna moved ninth, level on 21 points with troubled Juve with a 1-0 win at Spezia thanks to Marko Arnautovic's penalty seven minutes from the end on the Italian Riviera.
Earlier Andriy Shevchenko earned his first point as Genoa coach with a largely uneventful goalless draw at Udinese.
Genoa have been without key striker Mattia Destro for a month and the away side lacked firepower despite having more of the ball at the Dacia Arena.
On Wednesday his team host Milan, where the former Ukraine striker scored 175 goals and won one Serie A title and the 2003 Champions League.