The 19-year-old only made the switch to the middle of defence earlier this season to deputise for injured skipper Phil Jagielka, having previously operated mainly as a right-back.
Speaking on Sky Sports' Goals On Sunday, Martinez said: “Remember, John is only 19 and Saturday was probably only his ninth game as a centre-half.
"At Barnsley he always played at right-back, but he's got that knack of knowing where to be.
"He's got a really good defensive brain and fantastic defensive intensity - but then he's got the best distribution I've seen in a centre-half."
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