Fahrmann has usurped the experienced Timo Hildebrand as Schalke's first-choice goalkeeper this season, and a series of impressive displays from the 25-year-old have helped the club up the table.
Schalke have made the most of Bayer Leverkusen's recent woes to move to third, and they sit just one point behind their second-placed rivals Borussia Dortmund.
Fahrmann kept a clean sheet against Dortmund with a string of fine saves in Tuesday's Revierderby and Keller feels he has the capabilities to make Joachim Low's squad for Brazil.
"If you look at how he (Fahrmann) has played in the past few weeks, he's been on a very high level, consistently without mistakes," Keller said.
"He definitely doesn't have to hide behind any goalkeeper, except for Manuel Neuer maybe.
"But these are questions that Joachim Low has to answer, not me, he will have noticed for sure.
"Hans-Dieter Flick (Germany assistant coach) was also present in Real Madrid when he produced a great performance.
"I mean Joachim Low will certainly take a good three goalkeepers with him to Brazil."
Fahrmann will next be in action on Friday when Schalke host Hertha Berlin.
