Fahrmann was absent during a session at Schalke's training camp in Doha on Friday after experiencing discomfort in his right knee.
The German subsequently went to hospital for scans on the issue and there were initial fears that he had torn his cruciate ligament.
However, those worries were allayed with the results showing Fahrmann had suffered a strain.
No timescale has been put on a return, but Schalke expect the former Eintracht Frankfurt stopper to miss the start of the second half of the Bundesliga campaign.
"[Schalke's] number one picked up a knee injury during Thursday's training session and on Friday the diagnosis was made," read a statement on Schalke's official website.
"The keeper has strained the posterior cruciate ligament in his right knee.
"The injury isn't as serious as first feared. Despite this, the 26-year-old will miss the start of the second half of the season."
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