McCarthy has had an impressive season with the Latics and has been attracting the attention of the big clubs including Chelsea and Liverpool.
"I don't think he's reached his full potential with Wigan," said Martinez. "He's well aware of that. I think James knows he can progress a lot at Wigan for another period, especially if we stay up.
"He has an important part to play in that, because the reason James is playing in the manner he is at the age of 20 is that he's a winner. "When you're a winner you don't want to face relegation and you don't want to lose a challenge. Happily, that feeling runs right through our squad. "Nobody is going to stop his step to the next level eventually. I have no doubts James is going to play at the highest level of European football. "He's going to be involved in Champions League football because he's got that talent - but it's the timing that is so important. "It has to be right. He was playing in the first team at Hamilton at the age of 15 and did not leave them until he achieved his potential with them. "I feel he has a little bit longer here to experience a few different things before he moves on. "But we'll be the first ones to be proud and delighted for James when he moves on because he's been a real ambassador for Wigan."
