Chelsea defender Levi Colwill faces a long spell out of action after undergoing surgery on an anterior cruciate ligament injury.
The club said Thursday that England international Colwill had "successful surgery" after "medical assessments confirmed surgery would be the required course of action."
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The injury occurred as Chelsea reported for preseason training this week, following their victory over Paris Saint-Germain in the Club World Cup final three weeks earlier.
Chelsea manager Enzo Maresca earlier expressed concern about Colwill's injury.
"It was in the first session on Monday, just in the last minutes of the session," Maresca said in a news conference. "He felt something and, as I said, we need to wait and then we see.
"We don't know how long he is going to be out. I spoke with him yesterday and I told him that if we achieve what we achieved last year, it is also because of him -- and he was a main player for us."
Colwill, 22, was a key performer for Chelsea last season, making 43 appearance in all competitions as the London side finished fourth in the Premier League and won the UEFA Conference League as well as the Club World Cup.
Chelsea kick off their Premier League campaign at home to Crystal Palace on Aug. 17. The Colwill blow could possibly open the door for Jorrel Hato, Chelsea's new signing from Ajax.
Information from PA Sport was used in this report.
