Chelsea forward Christopher Nkunku has undergone knee surgery that will rule the French international out for an "extended period" the Premier League club said on Tuesday.

Nkunku, 25, joined the Blues in June from RB Leipzig for a reported £63 million ($80 million).

The BBC and The Telegraph reported that Nkunku could be out for up to four months.

He was expected to spearhead a new-look Chelsea this season under the management of Mauricio Pochettino.

The news comes as a hammer blow just days before they begin their Premier League campaign at home to Liverpool on Sunday.

In a statement, Chelsea said: "Nkunku has sustained a knee injury which will rule the forward out for an extended period.

"The 25-year-old has undergone an operation and will now begin a rehabilitation programme with the club’s medical department."

Nkunku, who missed last year's World Cup finals due to a left knee problem, suffered the injury during a pre-season friendly against Borussia Dortmund in Chicago last week.

Pochettino described the pitch at Soldier Field as "not perfect" but conceded that is the risk in playing a money-spinning tour of the States on surfaces designed for multi-sport use.

Chelsea could now be forced into the transfer market to recruit more firepower.

A return of just 38 goals in the Premier League last season contributed to a 12th-placed finish -- their lowest since 1994.

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