Chelsea defender Levi Colwill faces a long spell on the sidelines with a knee injury.
The centre-back suffered cruciate ligament damage in training on Monday and has had surgery, the club confirmed on Thursday evening.
The squad returned for the start of pre-season training on Monday.
And head coach Enzo Maresca revealed at a news conference on Thursday morning: “We had a problem the last two days with Levi Colwill and we don’t know how long he’s going to be out.
“It was the first session on Monday. Just in the last minutes of the session, he felt something.
“Hopefully not too long, but for sure it’s a problem for us.”
Asked about the nature of the injury, Maresca said: “We need to wait. We don’t know yet.”
However, the extent of the problem was later announced by Chelsea.
In terms of possible replacements for the England international, Chelsea recently signed Jorrel Hato from Ajaxand also have the likes of Tosin Adarabioyo, Trevoh Chalobah, Wesley Fofana, Josh Acheampong and Benoit Badiashile.
But Maresca did not rule out a signing being made should Colwill be sidelined for a lengthy period.
“It depends,” he said. “We need to wait and then we’ll see.”
The Blues will face German side Bayer Leverkusen in a pre-season friendly at Stamford Bridge on Friday evening.
Their Premier League campaign will begin with a home match against Crystal Palace on 17 August.
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