Zouma penned a five-year deal with the Blues in January, but he remained with St Etienne for the rest of the French season.
"When I had a phone call from Mourinho, I thought someone was imitating him," the 19 year old told France Football magazine.
"First, I could hardly talk. It is very flattering to have a call from one of the best coaches in the world.
"He told me I have an interesting profile and he wanted to do with me what he did with Raphael Varane at Real Madrid. Thanks to him he became one of the best French defenders.
"When you hear that, you reply yes immediately. Being one of the best French defenders is an objective for me."
He admits that Man United were one of a number of clubs trying to sign him.
"Monaco was the first to call, with the sport director. I thought about it, but with Mr Mourinho, that is a strong signal.
"I know it is a risky choice, but I wanted to catch that train. I didn't know if I would get that opportunity again.
"By signing at Chelsea I am keen to reach a new level."
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