A statement on Schalke 04's official website has suggested that Klaas-Jan Huntelaar could require surgery to repair damage to his right knee.
The 30-year-old has not featured for the German outfit since August after injuring his knee, which could require an operation to correct the issue.
"Klaas-Jan Huntelaar suffered deterioration in his right knee after a training incident on Monday," read the statement.
"This resulted in the 30-year-old having an MRI examination on Tuesday. Whether the injury is treated conservatively or surgery is necessary, all parties will decide in the coming days."
The Dutchman had initially been expected to return this weekend.
