Bayern Munich and Germany goalkeeper Manuel Neuer will be out until January after surgery on his broken left foot.
The 31-year-old World Cup winner sustained the same injury to the same foot in April and suffered a repeat in training on Monday.
Bayern are in the same Champions League group as Celtic and Neuer will miss their remaining five games.
“The operation went perfectly, which is the most important thing for now,” said Bayern chairman Karl-Heinz Rummenigge.
“We are now looking to the future. Manuel will be back to his previous best and available in January.”
Bayern, who began their Champions League campaign with a win over Anderlecht, next face Paris St-Germain in France before hosting Celtic on 18 October.
Sven Ulreich, a 29-year-old who has made 12 competitive appearances for the German champions since signing from Stuttgart in 2015, is now favourite to start those games.
Neuer picked up the original injury during the Champions League quarter-final second-leg defeat by Real Madrid in April.