The Holland international had to leave the pre-season Audi Cup match following a challenge from Taye Taiwo.
But, after having a scan this morning, Robben claimed his problem was not thought to be severe.
"The injury is not so bad. It is a capsule injury. There is a small crack. We now look from day to day for what happens next," the 27-year-old said.
Bayern are already having to do without another star player, France international Franck Ribery, who has an ankle injury of his own.