LONDON: Chelsea's preparation for the new season has suffered a major setback with the news that French international striker Christopher Nkunku has had a knee operation and will be out of action for an "extended period."

Nkunku, who cost 52 million pounds (66 million US dollars) from RB Leipzig, suffered the injury playing in a pre-season friendly against Borussia Dortmund in Chicago and the club has now confirmed he will be out of action for a considerable time, reports Xinhua.

"Nkunku has sustained a knee injury which will rule the forward out for an extended period," the club confirmed in a statement, adding that "the 25-year-old has undergone an operation and will now begin a rehabilitation program with the club's medical department."

The club now has to decide whether they need to dip back into the transfer market to sign a replacement.