Martinez said that McCarthy, who followed his old manager from Wigan Athletic to Everton in the summer, keeps things ticking over in the midfield, which allows other players to take more risks in attack.
"James is one of those players you sometimes don't appreciate but he's always there making sure the team has the right balance and what it needs," Martinez told the Liverpool Echo.
"He allows others to shine. He can adapt to different situations during a game when you need him to because of the tactical awareness that he has.
"At the weekend you saw that influence in the manner that we were able to get men forward, and give us the momentum to make it hard for Aston Villa to keep a clean sheet."
Everton's next game is away to Tottenham Hotspur on Sunday.