The £3m signing from Barnsley has made 18 appearances for the Blues this season and started the last five matches in place of the injured Phil Jagielka. "John Stones has been phenomenal," Martinez told the Liverpool Echo.
"His level of performance, in such a demanding league, at 19 and facing players of different types and calibre has been exceptional.
"He went to the Britannia Stadium and coped with a direct approach and the physicality of their dead ball situations.
"There is no doubt in my mind that John Stones will become the best centre-half in England and he is reaching a level that I wouldn't expect him to at such a young age."
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