Everton midfielder and Chelsea target has a buyout clause in his contract, which could lead to him leaving Goodison Park this January, or later in the summer.
The club has refused to comment on reports that the midfielder has a £22m release clause in his contract, but the Merseysiders insist they never divulge confidential arrangements of player contracts.
However, there is a fee that could trigger a deal - but it is believed to be higher than £22m, around £27-£30million.
The 25-year-old Belgian has been linked with a move to Chelsea and Manchester United since the beginning of the season.
He has emerged as a central figure in Everton's impressive season, scoring nine goals to date.