The report from talkSPORT claims Isco is aware of interest from several top clubs, but is happy to stay with the Spanish side for now. Manchester United and Manchester City have been linked with a move for the 20-year-old, who has impressed in Malaga's Champions League campaign this season.
The Spanish playmaker is in his second season at Malaga, and played an important part as his club secured qualification for the Champions League for the first time in the club's history last term.
And Casado insists Malaga will be able to hold onto the former-Valencia star, despite interest from United and City.
"Isco? There is a clause of €21m, but the guy is very happy to be in Malaga, he is a son of our city," Casado told Sky Italia.
"He is a player who is 'in fashion' and courted by many teams, but for now he is happy with us."
Isco starred this week as Malaga held AC Milan to a 1-1 draw in the San Siro - the Spaniard responsible for a perfectly measured through-ball for Eliseu to stab Malaga ahead before Alexandre Pato equalised for the Italians.
City and United have both scouted the Malaga player, and the €21m release clause has sparked speculation one of the Premier League suitors could look to move for Isco in January.
But it now seems the young midfielder is content to stay put in Spain, at least until the end of the season, which means City and United may have to explore alternatives options for the January transfer window.