Manchester City defender Aymeric Laporte has to wait until January or February, coach Pep Guardiola says.
The 25-year-old French center-back was operated on his right knee after being sent off in a 4-0 win over Brighton on 31 August.
In a challenge with the defender of Brighton, Adam Webster, he injured his cartilage and the lateral meniscus.
At Laporte's absence, Guardiola said, "Not for long – five months, six months – next year, January or February."
The defender played 35 league games last season, with Bernardo Silva the only player in the outfield of City, who played more.
His injury leaves City only with Nicolas Otamendi and John Stones as the center-back, after Vincent Kompany replaced in the summer Anderlecht player-coach.
Stones was able to play in Norwich City on Saturday at 17:30 (CET), and Guardiola said the English central defender had "trained well".
Guardiola supports the idea of a city transfer
In the meantime, Guardiola said his club's proposal to set different deadlines for national and international transfers for the Premier League was "a good idea."
Many clubs, including City, were unhappy with the three-week gap between closing the English transfer window and the rest of Europe, believing it would give the rivals an unfair advantage.
At their meeting in London on Thursday, the Premier League leaders discussed whether they should resort to the end of August, instead of closing the window on the Thursday before the start of the season, as was the case in the last two seasons.
City proposed a compromise solution, with the time limit for transfers within England remaining, but allowing clubs to sign players from other European countries for the remainder of August.
Guardiola said, "It's a good idea.
"I want to close the window the day before the start of the season, all managers want to start with the players we work with, but the other clubs have three weeks, so maybe that's a solution."