Bayern Munich midfielder Corentin Tolisso suffered an anterior cruciate ligament tear in Saturday's 3-1 win over Bayer Leverkusen.
The club also confirmed that right-back Rafinha is ruled out "for a number of weeks" after sustaining ankle ligament damage in the Bundesliga match at the Allianz Arena.
Tolisso, part of France's World Cup-winning squad in Russia, levelled for Bayern after the visitors opened the scoring from the penalty spot. But was taken off in tears just before half-time after picking up the injury.
Meanwhile, Rafinha was hurt under a challenge by Karim Bellarabi which saw the winger receive a red card just seven minutes after coming on as a second-half substitute.
Bayern president Uli Hoeness called for Bellarabi to receive a three-month ban for the tackle he described as an "assault".
Colombia international James Rodriguez, who scored in Saturday's win, posted a message of support for his team-mate Tolisso following the win on his Instagram story, saying: "Comeback stronger, brother".
Arjen Robben scored Bayern's other goal as new boss Niko Kovac continued his 100 per cent start to life in Bavaria.
Bayern Munich's next game is a trip to Portuguese side Benfica in their Champions League Group E opener on Wednesday.