The Turkey international has already missed two months of action due to the knock, but could face an even longer spell on the sidelines
The 22-year-old midfielder left Borussia Dortmund for Real Madrid in the summer transfer window, but has yet to make his first appearance for the Santiago Bernabeu side due to a knee injury.
Sahin picked up the knock in training when Madrid prepared for their pre-season tour to USA in July and has since been sidelined.
The left-footed midfielder was initially expected to be out of action for only three weeks, but his comeback seems far away at the moment after he has endured a recovery setback.
Madrid fear that Sahin will be unavailable for at least two more months, meaning that he will miss up to eight Liga matches and the opening three games of the Champions League group stage.