AC Milan vice-president Adriano Galliani admits he was shaken by the attack from Napoli ultras on Sunday night.
He said: "It was 15 minutes of brutality. If the windows had not stayed strong, it could have become very serious. Then the police came, but I do not want to talk about it."
The Mayor of Naples, Rosa Jervolino, admitted: "The aggression committed last night against the managing director of Milan was very serious and as mayor of the city I apologise to Galliani.
"We do not forgive the fact that it was committed by a few dozen people out of thousands of spectators who were at the stadium.
"Such violent aggression is unworthy of a civil city."