The Rossoneri suffered their second consecutive Serie A loss after a 4-3 away to Fiorentina, the first at San Siro in the league this term.
Alessio Romagnoli’s glancing header on a short corner had put them in front, but it was turned around by a Gianluca Scamacca screamer from distance, another Scamacca effort that deflected in off Simon Kjaer and Domenico Berardi’s angled drive.
“We lacked sharpness, because we made too many mistakes. We had so many half-chances that we didn’t take because of the wrong choices.
“When we did take the lead, we didn’t hold on longer than three minutes, and those are the crucial moments of the match. Clearly, something didn’t work.”
Pioli made some rare squad rotation choices in midfield and they did not pay off, Tiemoue Bakayoko and Ismael Bennacer providing no protection in the first half.
Milan have now conceded seven goals in two Serie A games, losing 4-3 to Fiorentina last week.
“I didn’t see a lack of concentration or determination. I think mentally we just weren’t sharp and made the wrong choices.
“Our approach to defending does require one-on-one situations, it’s just that we lost several of them today against quality players like Scamacca and Berardi.
“It’s never just about the defence, we gave the ball away too cheaply and certainly shouldn’t have conceded the third goal.
“I don’t believe it was the wrong approach, we just made bad choices. We are aware of our strengths and our weaknesses, so we need to keep working. We had a very strong start to the season, then in recent games have fallen away a little due to various situations, but we can get back on track.
“This squad was built for two tournaments and we hope to remain in the Champions League for as long as possible. I used five players who weren’t starting in Madrid precisely so we’d have fresh legs and minds, but Sassuolo are a technically-gifted and quick side who created real problems.”
Pioli had a final tribute to pay for the ultras at San Siro, especially after so many of them travelled to Madrid for the midweek Champions League win over Atletico.
“I want to send a message to our Milan fans, who really were pushing behind us from start to finish. They never let us down and with them we will go far.”