Knox has taken heat for not capping McCarthy as a teenager for Scotland, with the midfielder now a Republic of Ireland international.
"James has got himself a great move to Wigan," Knox told the Scottish Sun. "Like everyone else, I want to see him do well.
"But I don't want anyone making mountains out of molehills over this. There have only been two players at 16 and 17 who I have watched in my entire life and been prepared to place money on them becoming international footballers.
"One was Giggs and the other was Rooney. I don't care what anybody says - if you're going to tell me that anybody at 16 or 17 is a certainty to play international football then I'm not well enough qualified to do that.
"How can people be sure now? I've seen players bought for £20million and you still can't be sure. There still isn't certainty.
"Let me be straight about this. James is a good young player who is capable of doing very well.
"With more experience, and being exposed to the Premiership, he has a great chance. If he's got it in his locker then he will do it. There's no question.
"But for someone to say they are 100 per cent sure - well, he knows his football better than me."