Goalkeeper Fahrmann strained the posterior cruciate ligament in his right knee during a training session earlier this month.
That means Giefer, who moved to the Veltins Arena from Fortuna Dusseldorf in the close-season, is set to make his Bundesliga bow for Schalke when they restart their campaign at home to Hannover on Saturday following the mid-season break.
"Fabian is a great goalkeeper and a good replacement for Ralf," Fuchs told Schalke's official website.
"Both are not only extremely strong goalkeepers, but also good footballers."
Schalke reached fifth position before the break and Fuchs claims their training has left them in good stead ahead of the second half of the season.
"The whole team is looking forward to playing Hannover," he added. "We have worked very well in preparation.
"The 10-day training camp in Qatar was very intense, then we have continued our programme in Gelsenkirchen. We are definitely ready for the start."
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