The Swede was in unstoppable form before suffering a hamstring injury against Napoli on November 22, scoring 10 goals in six league games – a tally that remains the joint-highest in the league with Cristiano Ronaldo of Juventus and Inter’s Romelu Lukaku.
Milan have coped admirably in his seven-match absence, and on Wednesday became the first team in Serie A history to score at least two goals in 14 consecutive league matches.
But the on-field leadership qualities of the 39-year-old have been missed, as well as those of experienced centre back Kjaer.
While Milan continue to find the net with relative ease – their tally of 27 strikes is second only to Inter – the goals they shipped in the last two games have left Pioli concerned.
“Today we again conceded twice but didn’t give much away,” the coach told DAZN.
“The goals we’re conceding are becoming a bit too much. Today we didn’t mark well enough or cover the space for the two crosses.”
Milan remain unbeaten in Serie A since losing to Genoa on March 8, a run of 24 games, their best record since 1993.
But that streak will face a stern test against a Sassuolo side that has punched above its weight this season.
Coach Roberto De Zerbi has won many admirers for his attractive style of football and he has matched that with results this season, leading them to fifth in the standings, just five points behind Milan.
However, Sassuolo have also dipped in form lately, winning one of their last four games and scoring just two goals in that time.