NEW CHELSEA FORWARD Christopher Nkunku is set for an “extended period” on the sidelines after the Blues confirmed their summer signing has undergone knee surgery.

The 25-year-old France international moved to Stamford Bridge from RB Leipzig for a reported €60 million (£52m) but suffered an injury in Chelsea’s pre-season draw against Borussia Dortmund in Chicago.

“Nkunku has sustained a knee injury which will rule the forward out for an extended period,” Chelsea said in a short statement confirming the attacker had gone under the knife.

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

While the club have offered no official timeframe for recovery, some reports have suggested the former Paris St Germain forward could be out until December.

Nkunku is one of seven summer additions at Chelsea and looked set for a big role under new head coach Mauricio Pochettino before his injury setback.