Portsmouth defender Zak Swanson could be out for up to four months after undergoing surgery.
The defender has not featured since their 6-1 defeat by QPR before the international break due to a knee injury.
Last week Blues boss John Mousinho revealed that both he and the club's medical staff were weighing up options to either risk playing him for the rest of the season or to have surgery.
The full-back will now miss 12 to 16 weeks as he recovers.
Swanson, who is out of contract at the end of the season, joined the Blues in 2022 having come through Arsenal's academy.
Since his Portsmouth debut he has played 111 times and scored six goals and was part of the side that won promotion from League One in 2024.
Mousinho has also confirmed there is a chance midfielder Ebou Adams could make his return to the side this weekend when Portsmouth host Leicester in a huge game at the bottom of the Championship.
Adams suffered an injury in that heavy defeat by QPR on 21 March and has not played since.
"He's got a few bits to tick off tomorrow and if he does that then I think he'll be available but we're still sort of fifty-fifty with him," Mousinho told BBC Radio Solent.
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