With competition for places at the top clubs more intense than ever, the same fate could well await McCarthy if he were to leave the DW Stadium.
Martinez said: "If you don't play you'll never fulfil your potential.
"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.
"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."