The Republic of Ireland international has become a key player for the Latics since joining the club from Hamilton in July 2009 and scored both goals in Saturday's 2-2 draw with QPR - his first for almost two years.
Wigan are no strangers to losing star players to other clubs having seen Leighton Baines, Antonio Valencia, Wilson Palacios and Victor Moses all depart for other Barclays Premier League clubs, the latter joining Chelsea earlier this season.
Martinez would certainly not be surprised if McCarthy was the latest player to attract interest, saying: "That's the nature of the game.
"Manchester United lost (Cristiano) Ronaldo so I'm not going to be concerned about that. I'm pleased to see him improving, enjoying his football, taking responsibility and putting in performances to get good results for Wigan Athletic.
"James McCarthy's got the potential to play anywhere in the world and whenever he's ready we'll never stop him. But James McCarthy is at a very good club now and we want top players to stay here for as long as we can."
Moses has made an encouraging start to life at Chelsea but has only started four Barclays Premier League games while Palacios has struggled to make an impact, first at Tottenham and now Stoke.
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."
Saturday's draw made it three in a row for Rangers under Harry Redknapp but also ensured they set an unwanted Premier League record of 16 games without a win to start a season, leaving them eight points from safety.
McCarthy gave Wigan the lead after 19 minutes but Ryan Nelsen headed his first goal for QPR seven minutes later and it looked like they might get that long-awaited win when substitute Djibril Cisse put the visitors ahead in the 71st minute.
They could not hold on, though, and McCarthy earned Wigan a point by poking in the equaliser three minutes later.
Rangers defender Clint Hill said: "I think we started the game really well. Wigan came into it and we caused our own problems really by giving sloppy balls away and not marking correctly but then Wigan came at us again in the second half, threw everything at us for the first 20 minutes.
"We managed to get through that storm and score a good goal through Djib and then unfortunately again we shot ourselves in the foot a couple of minutes later by not picking up our runners, the shot went through a few legs and ended up in the back of our net.
"It's just the way it's going for us at the moment. We've got to try to take the positives. It's three unbeaten - obviously we'd like wins but we've got to start somewhere.
"That's two away we're unbeaten as well, and I think we had one of the worst records for away games so that's another positive. We've just got to try to get that first win against Fulham on Saturday."