It was expected that the winger would return to training at the end of last week, but he has prompted suggestions that the knock could be more serious than first feared.
"This is not, unfortunately, the way it should be. But it is difficult to tell when [I will have recovered]. We are trying everything. But it's definitely nothing too bad," Robben told Bild.
Bayern continue their Bundesliga campaign against Fortuna Dusseldorf on October 20.