Chelsea striker Christopher Nkunku will be out for “an extended period” after undergoing surgery on his injured knee.
The France forward, 25, suffered the problem during the first half of Chelsea’s pre-season friendly against Borussia Dortmund in Chicago last week.
Blues boss Mauricio Pochettino said at the time he hoped the injury would not be a “big issue” for his £52m striker.
Chelsea, who host Liverpool in their Premier League opener on Sunday, have not specified a length of absence.
However, some reports have suggested the former Paris St-Germain and RB Leipzig forward could be out of action until December.
Nkunku had impressed during Chelsea’s pre-season tour of the United States, scoring three times in five games and displaying a promising link-up with fellow summer signing Nicolas Jackson in attack.
He had netted 58 goals in the past two seasons for Leipzig before finalising a switch to Stamford Bridge in June.
His unavailability is a blow to Pochettino’s new-look Chelsea as they bid to improve on a poor goal record that saw them score just 38 times in the Premier League last season on the way to a 12th-place finish.