Tomori, 21, has already made 15 appearances for the Blues this term, helping turn Frank Lampard's men into title contenders.
And Zola feels former Blues manager Maurizio Sarri - as well as himself - might have missed a trick.
He told the Evening Standard: "Tomori is one of the players. Last year we had him in the pre-season and we couldn't see all the qualities that he is showing now.
"That is a surprise and a big lesson for us and me personally as well. They are going beyond my expectations."
Zola added: "Mason Mount is not surprising me because I saw the videos of him playing in Holland and Derby. He needed to adapt but there was always a big potential.
"I'm not surprised about Tammy [Abraham]. When we had him he was a raw talent but I loved the attitude and positivity. They have all got a great attitude."