The 44-year-old had an operation on a salivary gland last November but it is believed that the problem has now returned and the Spanish champions have offered their backing to the coach of their fiercest rivals.
"Real Madrid want to express their support, love and affection to Barcelona coach Tito Vilanova and hope that he makes quickest recovery," read the statement on the club's official website.
"Real Madrid extend this support to your club and your family."
Barca have yet to officially validate the reports but the Catalan club closed Wednesday's training session to the media.
The Blaugrana top La Liga after taking 46 points from a possible 48 - the best start made by any Spanish side in Primera Division history.