The Cologne right-back has been a virtual ever-present for Fulham since his switch to Craven Cottage in the summer.
Jol said: "Sascha Riether has done a great job for us. He's enthusiastic, mobile and aggressive.
"He was playing for a club who got relegated and I thought that maybe this was the chance for us to get him under good circumstances.
"I only wanted him on loan because you never know [how they will do].
"If he carries on like this we will make our mind up and try to extend it or buy him."