Whelan was speaking to journalists during a promotional visit to the K Club in Kildare when he took a call from Everton boss Roberto Martinez regarding the 22-year-old Republic of Ireland international.
However, Whelan has told the man who brought the FA Cup to the DW Stadium last season that he will have to come up with at least £14million, something he expects will happen if the Toffees sell Belgian midfielder Marouane Fellaini to Manchester United.
Whelan told the Irish Independent: "Roberto is chasing James. If Everton sell Fellaini, they are going to come in again for our boy.
"It depends whether Fellaini goes today. I have heard that he's definitely going.
"If Fellaini goes, Everton are going to get their £30million or whatever. Roberto knows what James is valued at, so they've got to come back in with the right amount. You've got to be talking £14million or maybe a little more."
McCarthy has not joined up with the Ireland squad yet as a result of the interest in him, although it remains to be seen whether or not a deal materialises before Monday night's 11pm deadline.
Whelan added: "I would never stop a footballer going up to a bigger club, or to the Premier League. I know what it's like if you stop a footballer going to the club he wants to go to; you never get the same response or loyalty out of him ever again if you stand in his way."
Newcastle too have expressed an interest in McCarthy, although they have as yet come nowhere near matching Wigan's valuation of the player.
In any case, the Magpies' pursuit could be contingent on what happens with France international Yohan Cabaye, with time running out for Arsenal to improve upon the £10million bid they had rejected earlier this summer.
Source: PA
Source: PA