The Senegal defender, who knows Lukaku well from their time playing against each other in Italy, suggested that there could have been issues in the striker’s ‘private life’ that caused on-pitch struggles.
Speaking during his press conference on Wednesday, Koulibaly said: “Romelu is a good player, a nice player, a talented player.
“He is very strong and I was used to playing against him in Italy – I think he didn’t like to play against me!
“Last year I think maybe he didn’t have the confidence he needed to play like he wanted. Maybe something else, I don’t know.
“Football is not all about the pitch, it is also your private life and everything else, so maybe it was something that didn’t work with him over here.”
Koulibaly added: “We have a lot of talents, a lot of good players, a lot of winners, a lot of experienced players, a lot of young players. I wish Lukaku a good season but I wish Chelsea a better season.”