Keane is working with McCarthy as Republic of Ireland assistant coach.
"I've seen James a number of times and that will come with age," he said. "I was in the same boat myself when I was at Nottingham Forest and you're surrounded by senior players and then you get a move to United.
"You're working with Bryan Robson and you mature into the job so that's a process James will go through. It's nothing unusual for a young player."
Keane added: "If he's playing week in, week out for Everton, the benefits will be for Ireland as well - but don't be trying to make him something he's not, that will take a bit of time.
"He might never be a ranter or raver but sometimes it's not about that. It's about making more demands of his team around him or demanding the ball in certain situations.
"All that will come of age. Don't be impatient with him."
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