Wigan are adamant that he will not be leaving the DW Stadium on the cheap, even though they were relegated from the Premier League last season.
New Everton boss Roberto Martinez was instrumental in shaping the career of McCarthy, who is now emerging as a key man for Ireland after a stuttering start to his international career.
The Spaniard will be keen to make a marquee signing at his new club, although the £20m fee would break their transfer record as they aim to push on from their impressive recent seasons under David Moyes.
If Everton could find the cash to mount a bid, the chance to link up again with Martinez and fellow Ireland internationals Seamus Coleman and Darron Gibson may appeal to McCarthy.
Roberto Mancini was also believed to have been a huge fan of McCarthy but his sacking appears to have finished any chance of the Wigan man moving to Manchester City.
Arsenal are also believed to be in the running for the Glasgow-born star while Wigan have already rejected an £8m bid from Fulham for the 22-year-old, who has three years remaining on his contract.