Koulibaly, 31, had long been linked with a move to England's top flight before eventually arriving from Napoli in a deal worth £33million over the summer, and scored his first goal in the derby against Tottenham in his second game.
However, a dismal display against Leeds followed, where he was sent off during a 3-0 defeat, and has faced criticism from some quarters as the Blues made an uneven start to the league season.
Ahead of Chelsea's Champions League opener against Dinamo Zagreb, Capelli told Sky Calcio Club: "I would like to talk about the difficulties of Koulibaly at Chelsea.
"In Naples, he was doing everything by himself. His problems now depend on the speed with which they play in England.
"It’s the same pace used by Udinese to give Roma fits. It’s something that must make us Italians reflect."
Koulibaly is expected to start tonight’s Group E game in Croatia, alongside Wesley Fofana and Cesar Azpilicueta.