Farfan remained in Germany while the rest of the Schalke squad spent the last week preparing for the second half of the season at a training camp in Qatar.
He has been following an individual training plan, but Stevens admits he is not making very good progress.
"Unfortunately, he cannot yet train with the team and I don't know when he will be able to train with the team again," said Stevens on his club's website.
"I assume he will not be available for our first game of the second half of the season against Stuttgart, unless a miracle happens."
Lewis Holtby has also been ruled out of that game, but Stevens is otherwise pleased with the players he has had available in Doha.
"We set the objectives for the training camp in advance and we have managed to fulfil them," said Stevens.
"I am very pleased with the way the preparations have gone.
"We have worked on a few things in the training sessions and the players have been pulling their weight excellently in super conditions."
This did not prevent Schalke from losing their final warm-up game against Al Sadd, but Stevens is not reading too much into that result or the performance.
"Some of our players were a bit tired after the training session beforehand," he said.
"Therefore, they were not able to play to the best of their ability.
"The result of a friendly match is not the be all and end all, what is important is what we got to see during those 90 minutes."