Hannover's Stindl undergoes ligament surgery
Stindl was injured during the 2-2 draw at Spain's Levante in the Europa League last week and was taken off.
"Everything has gone well," the club said in a statement following the player's surgery in Switzerland. "But it is still unclear how long he will be out of action."
Hannover have qualified top of Europa League Group L for the next round of the competition.