Van Dijk had been at the top of the list, but now Napoli have made it known that they could be ready to do business over Koulibaly – following their signing of Vlad Chiriches from Tottenham Hotspur.
Both 24, Koulibaly had an excellent debut campaign in Serie A with Napoli last term, featuring in over 20 games for Rafa Benitez - whilst Van Dijk has emerged in recent seasons as one of Celtic's key performers.
Celtic have been looking for more than £10million for van Dijk, whilst Napoli are believed to be looking for just short of that for their man.
Southampton want another centre-half despite the recent loan signing of Steven Caulker, and Everton want another body having recently lost out to West Ham United for Juventus' Angelo Ogbonna, and there is also the realistic possibility they could still lose John Stones this month.
Both clubs are now pushing on with their interest in both players, but it remains to be seen if either can be brought to the Premier League in this window.