Borussia Dortmund striker Michy Batshuayi accused Atalanta fans of racist abuse on Thursday after his team's Europa League match in Lombardy.
The Belgian international, who is on loan from Premier League champions Chelsea, wrote on Twitter that he heard "monkey noises" from the Atalanta supporters.
He wrote: "2018 and still racists monkey noises in the stands... really?! Hope you have fun watching the rest of @EuropaLeague on TV while we are through... #SayNoToRacism. #GoWatchBlackPanther."
Atalanta were given a one-match stand closure last month, suspended for a year, after Napoli defender Kalidou Koulibaly was subjected to racist abuse.
Atalanta president Antonio Percassi later apologised to the Dortmund striker.
"I'll be honest, I didn't hear them. If they did happen, then I am very sad and absolutely apologise to Batshuayi. This should never happen," said Percassi.
German side Dortmund grabbed a 1-1 second-leg draw through a late equaliser from Marcel Schmelzer to progress to the last 16 with a 4-3 aggregate victory, after the 24-year-old Batshuayi had scored twice in the opening game.