Chelsea will decide whether to step up their pursuit of Dusan Vlahovic after striker Christopher Nkunku was ruled out for "an extended period" following knee surgery, sources told ESPN.

Nkunku, a 60 million ($65.7m) summer signing from RB Leipzig, suffered the injury in Chicago during Chelsea's final preseason friendly against Borussia Dortmund last week.

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The club said in a statement on Tuesday: "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."

Early indications are that Nkunku could be sidelined for at least three months -- news that will come as a major blow to new coach Mauricio Pochettino as he looks to make a positive start at Stamford Bridge.

Nkunku was one of Chelsea's better performers during their tour of the United States, scoring three times, as Pochettino looks to transform a side which netted just 38 goals in 38 Premier League games last season.

Sources told ESPN the club's need for more firepower may intensify their interest in Juventus forward Vlahovic, who is part of complicated negotiations involving a move away from Chelsea for Romelu Lukaku.

Although Chelsea have interest in Lukaku from Saudi Arabia, the Belgium striker wants to return to Italy. Juventus are willing to offload Vlahovic in a swap deal, sources told ESPN, but the clubs are far apart in their valuations of the two players.

Chelsea want around 46m for Lukaku, while Juventus have quoted 80m for Vlahovic. Pochettino is thought to be keen on Vlahovic, who has scored 23 goals in 63 games for Juventus since moving from Fiorentina in January 2022.