Zouma signed for Chelsea last month before being loaned back to St Etienne for the remainder of the season.
"In fact, first of all, there was a call from Mr. Mourinho. I had been warned. He spoke French, I understood everything. He clearly told me he wanted me. I immediately jumped at the chance," said Zouma.
"The two clubs made contact, they agreed in the final moments of the transfer window. Everyone is happy, it's only happiness."
Mourinho told Zouma he'd help him like he worked with Raphael Varane at Real Madrid.
"It's true, he told me about Varane on the phone, he cited him as an example. José Mourinho is one of the greatest coaches in the world. And I've been a fan of Chelsea since I was little."