The centre-back was signed from Napoli earlier this summer after years of dominating in the Serie A.
The likes of Manchester City and Manchester United were previously interested in the Senegalese star, but Napoli’s asking price proved too much.
However, with Koulibaly getting on and his contract running down, he was finally sold to Chelsea this summer for around £30m.
The stopper had a decent start to life in England, but he was then shown a red card in Chelsea’s 3-0 loss to Leeds United.
Since his return, he has not looked at his best, with Chelsea as a squad struggling defensively.
Now, former England boss Fabio Cappello has explained why Koulibaly is struggling in the Premier League, pointing towards the speed of the English game as a major factor.
“I would like to talk about the difficulties of Koulibaly at Chelsea,” Capello told Sky Calcio Club.
“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 now has a new defensive partner in Wesley Fofana, and Thomas Tuchel will be no doubt hopeful that the pair can form a good partnership alongside Thiago Silva to finally end Chelsea’s defensive struggles.
Chelsea play Dinamo Zagreb in the Champions League on Tuesday before facing Fulham on Saturday afternoon.