Heynckes admits that he is worried about the prospect of losing one of his most influential players at a crucial stage of the season.
"An initial diagnosis by FCB chief medical officer Dr Hans-Wilhelm Muller-Wohlfahrt suggests the Germany international has ruptured a group of adductor muscles in his right hip and now faces several weeks on the sidelines," a statement on Bayern's official said.
"This is obviously very bitter," Heynckes added.
"It's just such a shame for a young player who has played great this season."
The return leg takes place next Wednesday.