The left-back has been mentioned as Ashley Cole's successor next season.
But Bosz told the Daily Mail: "He's not ready. You cannot make the mistakes for Chelsea that he is making with us. It is not possible.
"He is progressing and he is not an old player, he is 23. He is fast and has fantastic technique. Going forward he is one of the best but in defence he has to learn a lot if he wants to play for Chelsea. He must be reliable."
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