Borussia Dortmund president Reinhard Rauball believes Mats Hummels is irreplaceable at the club.
The 24-year-old defender fell ill just before the midweek DFB-Pokal game with Bayern Munich after suffering with flu.
Subsequently, it was the Bavarians who progressed with a 1-0 victory, and Rauball thinks that the Germany international's withdrawal proved pivotal in the outcome of the result.
"We missed Mats hugely in three areas: as a stabiliser in the defence, in the game design and also in standards," Rauball told Bild.
Dortmund host Hannover on Saturday, and die Borussen coach Jurgen Klopp admitted that he is unsure whether Hummels will be fit for the game.
"We know that the flu comes quickly, but we do not know whether it will go fast again," he said. "Therefore I cannot tell you if Mats will be available to play on Saturday. We seek to ensure that he can play."
Hummels is widely regarded as one of Europe's most talented centre-backs, and he currently has 24 caps to his name for the Germany national team, scoring once.