Castelen to undergo knee surgery
Former Dutch international Castelen, who has only managed 15 games and one goal in his three years with the Bundesliga club after being plagued by injuries, picked up a knock on his right knee during training.
"I really do not know yet where I will find the strength for yet another comeback," the 27-year-old said on the club website.
"You just cannot have more bad luck," he said after only recently returning to full fitness.
Castelen underwent surgery on his left knee in October, missing most of last season.