Zouma, 19, is highly regarded in France and is expected to become a star of the future following his £12.5m switch to West London, though he will remain with Saint-Etienne for the rest of the season to continue his development in Ligue 1 before starting his Chelsea career in the summer.
The French Under-21 international follows Bertrand Traore, Nemanja Matic and Mohamed Salah to Stamford Bridge, but the impending arrival of Zouma will raise major question marks over the future of captain John Terry.
Terry, who has been ever-present for the Blues since the early 2000's, and has been a talismanic leader of the Chelsea defence throughout the reins of 9 permanent or interim managers. In the ever-changing landscape at Stamford Bridge, where managers and players come and go, Terry has remained as one of Chelsea's few constants.
But his time in West London could be coming to an end after 16 years of service. With the 33-year-old's contract set to expire in the summer, there is certainly no guarantee that Terry will get the extension that he is looking for, which would take him up to his retirement with the club he has spent his entire career with.
Zouma's arrival is timed perfectly for his introduction to the Chelsea first team, just as Terry may be leaving. With Branislav Ivanovic, David Luiz and Gary Cahill well positioned to compete for the two starting spots at centre back under Mourinho next season, there surely won't be room to carry Zouma and Terry as well.
Alternatively, Mourinho may look to cash in on Luiz, with Barcelona and Bayern Munich both reportedly tracking the Brazilian. A strong showing at the World Cup would almost certainly inflate the asking price for the former Benfica man, and it would also allow Terry to stay at the club as cover for the likes of Ivanovic, Cahill and Zouma - but the arrival of the Frenchman has certainly thrown Terry's long-term future at Stamford Bridge into doubt.
image: © Julian Mason