Levi Colwill will miss most of the season after undergoing ACL operation.
The Chelsea defender sustained the horror injury during the first training session of the blues of the preseason after their Triumph Triumph.
Manager Enzo Maresca hinted on Thursday that the club may have to go back to the transfer market to find a replacement.
The Italian greeted Colwill, one of his few leaders, as a key figure in the side who won the Conference League and Club World Cup last season.
Chelsea had only three weeks off between the end of the club World Cup on July 13 and the start of their preparations for the new season.
Colwill underwent a scan on Wednesday evening before he went under the knife on Thursday.
Speaking before the operation, Maresca said: “We had a problem with Levi Colwill in the last two days, that we do not know how long he will be gone.
“We will see, hopefully not too long, but it is certainly a problem for us.
“We have to wait, we don't know yet.
“It was in the first session on Monday, just in the last minutes of the session. He felt something and, as I said, we have to wait and then we see.”
Asked if they will sign a replacement, Maresca added: “It depends on it.
“We have to wait and then we see.
“But, as I said, we know each other from last year and you know how important Levi has been.
“I spoke to him yesterday and I told him that if we achieve what we achieved last year, this is also because of him – and he was a main player for us.”
Chelsea has signed teen Jorrel Hato from Ajax, who can play both in the middle of the back and left back.
Trevoh Chalobah, Tosin Adarabioyo and Benoit Badishile are their other current options.
Axel Disasi trains away from the main group as part of the Chelsea 'Bomb Squad'.
This is a developing story ..
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