Bayern Munich expect Alphonso Davies to be out for up to eight weeks after the full-back damaged ankle ligaments against Eintracht Frankfurt.
Davies was substituted after three minutes of Saturday's 5-0 Bundesliga victory at the Allianz Arena after rolling his ankle.
Head coach Hansi Flick said the 19-year-old has torn ligaments and is not expected to return until at least early December.
"He sustained a ligament injury. One ligament is fully torn, another is partially torn. We reckon he'll be sidelined for six to eight weeks," Flick told reporters.
"It hurts when a player of this quality is ruled out."
Davies is likely to miss most of Bayern's remaining matches in 2020, including their five remaining Champions League group games and the Klassiker with Borussia Dortmund on November 7.
The champions' win over Frankfurt, in which Robert Lewandowski scored a hat-trick before further goals from Leroy Sane and Jamal Musiala, moved them to within a point of league leaders RB Leipzig after five games.