The Wigan manager praised the 22-year-old Ireland international for his growing maturity on the pitch and said that he has massive potential
The 22-year-old midfielder joined the Latics in 2009 from Scottish side Hamilton Academical where he has been a regular fixture in midfield, and Martinez believes that the Republic of Ireland international could play 'anywhere in the world.'
"James McCarthy's got the potential to play anywhere in the world and whenever he's ready we'll never stop him," the Wigan boss told reporters. "But James McCarthy is at a very good club now and we want top players to stay here for as long as we can."
The former Swansea manager believes that the midfielder is developing as a player and taking more responsibility on the pitch due to the fact that he is playing regularly.
"If you don't play you'll never fulfil your potential," he said. "James has been through two very intense seasons, being very successful and having a massive role and the way he's developing as a player is something he would never do if he wasn't playing regularly.
He added: "This season is an important moment for him. He's starting to have a big role with the Republic of Ireland and you could see (against QPR) he took responsibility, he drove the team forward.
"And at 22 it shows the future for James is really impressive and that's why we put such a big investment in him from a very young age," concluded Martinez.