The Dutchman's thigh knock has led Netherlands coach Louis van Gaal to drop the Bedum native from his latest squad, with the winger instead remaining in Germany to continue his rehabilitation.
The former Chelsea man was absent from training on Thursday however, prompting rumours that his injury could be more severe than initially thought.
"This is not, unfortunately, the way it should be," Robben told Bild.
"But it is difficult to tell when [I will have recovered]. We are trying everything. But it's definitely nothing too bad," the 28-year old concluded.
With Bundesliga action resuming next week, Robben is unlikely to be fit enough for the Bavarians' upcoming trip to Fortuna Dusseldorf on October 20.