It was before a penalty kick that the incident occurred. As he stepped forward, several onlookers were heard screaming and making aplenty to Lukaku, which is clearly heard on a video that has become widely circulated on Twitter.
Lukaku on Monday posted a post on his social media, calling for power.
"I hope that football associations around the world react strongly to all forms of discrimination," he wrote, among others, and concluded his post with:
"Ladies and gentlemen: It's 2019 and instead of going forward we go backwards, and I think as a player we have to agree and make a statement about this to keep the game clean and enjoyable for everyone".
"Condemns what happened"Some of the people who condemn the event are Cagliari's clubbing:
"Cagliari Calcio condemns what happened at Sardegna Arena on Sunday night," one writes, among others, and continues:
"The club wants - once again - to emphasize that we want to identify, isolate and turn off the ignorant individuals whose shameful behavior completely goes against all the values that Cagliari Calcio stands for."
Despite the hurdle from the crowd, Lukaku put the penalty past the Swedish goalkeeper Robin Olsen, and Inter won by 2-1.