In the video, filmed during Thursday's Europa League match between the two clubs at the Emirates Stadium and posted on Snapchat, a fan can be heard using the word "n--", in reference to Koulibaly, with the word also written across the screen for part of the recording.
English Premier League side Arsenal have confirmed they are investigating a video, now circulating on Social media, which appears to show one of their supporters racially abusing Napoli central defender Kalidou Koulibaly.
Unai Emery is hopeful that Arsenal duo Granit Xhaka and Laurent Koscielny will be fit to face Napoli in the Europa League quarter-finals tonight, according to James Olley of the Evening Standard.
The Gunners say that they "utterly condemn" the language used.
To get the latest soccer news, text "SPORTS" to 22840. "We encourage supporters to report any incidents as they happen to stewards or via our matchday alert service".
The crowd at San Siro on Boxing Day were told over the stadium's public address system on a number of occasions to stop, but ignored those requests. "His focus is very big, he's thinking of Arsenal only".
"In the first half, we made a ideal match, then we had to defend and be united".
"I think we played well last night and it was important to win", he added. "When he's here with our supporters, he gives us all he can, not individually, but in the collective".