Serie A: AC Milan suffer heaviest defeat since 1998
Atalanta, who are in the Champions League knockout stages, rise to fifth in the table.
Milan had not lost by five goals in any competition since being beaten 5-0 by Roma in May 1998.
Pioli, the club’s third manager of 2019, said: “This cannot be the real Milan.
“[We] approached the match in the wrong way and you just can’t afford to do that against this team.
“Today’s performance was seriously insufficient. I expected a reaction from us in the second half but it didn’t come. In fact, we were even worse.”