Zouma has signed for Chelsea today, but will spend the rest of the season on-loan at St Etienne.
Mourinho said: "He is a St Etienne player until July, so there’s not much to say. He’s a young player in the profile of some Chelsea signings in the last couple of years. We like the profile. We think he can adapt to the Premier League.
"If we don’t do it now, we can’t do it in the summer because other clubs were strong on him. He will join us for pre-season and we will see if it is to stay or go on loan. He is a young player for a position where we need a future. Job done, and he stays at St Etienne.
"He is very young but he is playing already a couple of seasons in St Etienne and many international matches for the youth teams. He is a player with good potential. Physically he is ready but it is another thing to be tactically ready. So well done Chelsea, this is the last one in the transfer window. In almost every position we have the present and the future.
"No more signings. The squad is short. Even players like Kalas is needed. We have 20 for the Premier League and 18 for the Champions League. We are there. We let Michael and Ryan go to play. We can’t let others go. We need the squad."