The France international was substituted midway through the first half of the 1-1 draw with Borussia Dortmund in Chicago on Wednesday after falling to the ground clutching his leg.

Chelsea manager Mauricio Pochettino said Nkunku felt “something in his knee.”

“The doctors are checking him and I hope it is not a big issue,” Pochettino said.

“We hope he can be back quickly with the team. We need a few days to assess him.”

Nkunku, who joined from Leipzig for 52 million pounds ($67 million), has been a regular starter for Chelsea in its preseason games. If he does miss the start of the season, it may open the door for another offseason signing — Nicolas Jackson — to start up front.

Chelsea's first Premier League game is at home against Liverpool on Aug. 13.

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