The 20-year-old earned his first Bundesliga start for Bayern in the 4-3 win against Hertha Berlin on Oct. 4.
"He can't play," Flick told a news conference when asked about Richards. "He had some problems with his calf muscles.
"It took a bit of time, but it's getting better. But we must be careful and integrate him into team training again step by step"
Goalkeeper Alexander Nubel, meanwhile, is set for his competitive debut for Bayern. The 24-year-old joined on a free transfer from Schalke in the summer, but he has to line up behind club and Germany captain Manuel Neuer.
Following Sven Ulreich's transfer to Hamburg on the final day of the transfer window, Nubel is Bayern's second-choice goalkeeper and will get his chance in the cup match.
"I think that Manu understands that we will play Alex Nubel tomorrow," Flick confirmed, adding that he is determined to hand debuts to those who joined the club on deadline day and are eligible to play.