The 30-year-old suffered a setback on Monday after having recently returned to training following a knee injury.
Keller has confirmed that the striker will not feature for Schalke again this year.
"It doesn't really matter what the decision is on Klaas-Jan Huntelaar," Keller said.
"He can't play until Christmas. It doesn't matter if he decides to have an operation.
"He might recover without one. Both outcomes are possible. The decision has not been made yet.
"We didn't rush him back before he got injured. Usually an injury like his is fine after eight weeks. He felt good.
"He took part in everything and this setback was an accident. He collided with someone in a bad position. This would still have happened even if it was two weeks later."