The 30-year-old Fahrmann, who made his Schalke debut more than a decade ago and is also the team captain, was surprisingly dropped as first choice at the start of the year but returned to action a few weeks later when Nubel was sent off.
Tedesco, whose team were runners-up last season but are currently in 14th place, said on Tuesday Fahrmann would be his choice against the powerful City attack.
"We are in a comfortable position as far as goalkeeping is concerned because we have two outstanding keepers," Tedesco told reporters.
"The Champions League is an achievement of Ralf Fahrmann because of last season so he will be in goal tomorrow. Alexander Nuber will be back in goal on the weekend."
Schalke have their backs to the wall, having won just two of their last 10 league games. City are top of their league on goal difference ahead of Liverpool, who have a game in hand.
"We are the clear underdogs in the game, that much is obvious," Tedesco said. "But we deserve to be here because we advanced deservedly from a group of good teams.
"Tomorrow we have to have a perfect day if we are to get anything out of it. We owe it to us and to Schalke."
For defender Bastian Oczipka it is only through team work that the Royal Blues will have any chance against City's talented squad.
"It is nice for people to think that (they are much stronger) but City still have to prove they are stronger. We obviously want to counter that. We play a top team but we will try to use the chance we will have," Oczipka said.
"We have to solve it as a team and there is nothing more beautiful than such games."
