The Goodison chief sees no barrier to McCarthy's potential.
Martinez told evertontv: "Obviously when you've been working with a player previously - and with James I was doing that when he was at a very, very young age - you can only be surprised in the manner with which he coped with the situation.
"He is a credit in the way he behaves and the way he performs. All in all, he could potentially be one of the top midfielders in European football.
"He's a youngster with a real Everton make-up in that respect. He brings something that we haven't got and he adds to that special young generation that we have at the Club already.
"That really excites me in terms of looking at how he will develop in the next stage of his career."
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