Siegemund, the world number 32, sustained the injury during her second round match against Barbora Krejcikova at the Nuremberg Cup on Wednesday and was stretchered off the court.
She underwent surgery on Friday with a return likely to be early next year.
"We are trying to slow her down and say that 'You have to accept it as it comes'," Rittner told reporters in Nuremberg. "It does not really matter if it is January or February or March."
"She will not be out for just a month but for six."
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Siegemund reached a career-high ranking of 27 as recently as last month.
(Reporting by Karolos Grohmann; Editing by Hugh Lawson)
