The German sustained the problem in the 2-1 loss to QPR on Saturday and had to be forced off the pitch at half-time.
Manager Martin Jol said that Riether was unable to give anymore for the Cottagers after he was struck down.
"We have to assess it," Jol told reporters.
"The doctor came in and said he couldn't carry on so I had to change it."
Riether has travelled back home to Germany to be assessed by a specialist in order to find out the extent of the damage.