The 19-year-old has played a starring role for the Ligue 1 outfit over the last two seasons, attracting interest from some of Europe's top clubs including Manchester United and Liverpool.
But Chelsea manager Mourinho is preparing to up his interest in the French under-21 international, having been told by scouts that he is one of the standout young defenders in Europe.
Chelsea's defence has succumbed to the pressures of the Premier League recently, conceding six goals in their last two games.
Now the Special One is preparing to restructure his defence at Stamford Bridge with veteran John Terry, 33, nearing the end of an illustrious career. Doubts also remain over the future of David Luiz, who has been linked with a move to Barcelona in recent weeks.
The news could be bad news for David Moyes and Manchester United, who are in desperate need of defensive reinforcements.
Nemanja Vidic and Rio Ferdinand are both well in to their thirties and long-term replacements are needed to ensure the Red Devils stay amongst the elite in a competitive Premier League environment.
Zouma is currently in the middle of a 10-match ban in France after a horror tackle which broke the leg of Sochaux star Thomas Guerbert.