According to MMA Weekly:
Valentijn Overeem, brother of Strikeforce heavyweight champion Alistair Overeem, is injured and will be unable to fight Antonio “Bigfoot” Silva as planned on the Dec. 4 fight card on Showtime, said MMAWeekly.com sources. The nature of the injury was not disclosed.
…It sounds as if there aren’t too many fighters suitable for the Showtime telecast willing to step in on such short notice to face a fighter the caliber of Antonio Silva.
[UPDATE] – Valentijn Overeem suffered an elbow injury according to Sherdog.com:
Though the specific nature of Overeem’s elbow troubles remains unknown, he is reportedly awaiting the results of X-rays that will illuminate the severity of the injury.