French-born Senegal international Koulibaly, 28, has established himself as a key player in Naples and one of the world’s premier central defenders, but has been subjected to abuse on more than one occasion.
His first such experience came at Lazio in February 2016, when match referee Massimiliano Irrati temporarily suspended play after being made aware of abuse directed towards Koulibaly. Napoli went on to win the game 2-0, but it is an experience which has stayed with him.
“I realised some Lazio fans were making monkey noises whenever I touched the ball,” he said in the Players’ Tribune.
“It is impossible to know what you are supposed to do in this moment. There were times when I wanted to walk off the pitch to make a point, but then I told myself that this is exactly what they want.
“I remember thinking to myself, ‘Why do they do this? Because I am black? Is it not normal to be a black guy in this world?’
“You are just playing the game you love, like you have done a thousand times before. You feel hurt. You feel insulted. Honestly, it gets to a point where you feel practically ashamed of yourself.”
The defender praised the response of referee Irrati, who consulted with Koulibaly as well as coaches and officials before allowing the game to go on.
Despite Napoli finishing the game at the top of Serie A, Koulibaly left the pitch “very, very angry”, but sought out a young mascot that had asked for his shirt before kick-off.
Eventually finding him in the stands, he was taken aback as the youngster apologised for the abuse Koulibaly had received during the game.
“This really affected me a lot,” he admitted. “This little guy was apologising for I don’t know how many grown men. And this was the first thing he thinks about — how I was feeling.
“This is the spirit of a child. This is what we are missing in the world right now. I know that these incidents are not just happening because of skin colour.
“I hear what some fans call my teammates, too. They call the Serbian players ‘gypsies,’ they call even an Italian like Lorenzo Insigne ‘Neapolitan sh*t.’
“We need to do better. An incident happens, the clubs make a nice statement, and then it happens again.
“In England, we see how much has changed. When the person is identified, they are banned from the stadium forever. I hope that one day it is the same in Italy.
“But I also think of the people doing this. How can you make the people change? How can you reach them in their heart? I do not have the answers for this.”
Koulibaly is currently on international duty, at the Africa Cup of Nations with Senegal. He played 90 minutes as his side beat Tanzania 2-0 in their group opener, before Thursday's clash with Algeria and their final group game with Kenya on Monday.