The Toffees snapped up the 22-year-old from Wigan for £13 million on deadline day, reuniting the Republic of Ireland international with the manager that originally brought him to the DW Stadium in the summer of 2009.
The Spanish manager believes both he and McCarthy will benefit from having worked together previously, and has backed the youngster to prove his worth at Goodison park.
|FELLAINI SEALS LATE UNITED SWITCH
|8/1||James McCarthy is 8/1 with Coral to score anytime when Ireland face Sweden
"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.
"In any sort of environment, when you have a little bit of continuity and understanding the method of work, that always helps. More than anything, that can help with the settling-in period. That's important for any player.
"James is a veteran already at a very young age. Only at 22, he has played many, many games in the Premier League and is a full international with a terrific attitude and a great personality on the pitch.
"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."