Manchester City defender Alex Greenwood has undergone knee surgery on a medial collateral ligament (MCL) injury.
The captain was stretchered off in the first half of Man City's Champions League win over St Polten earlier this month.
Manager Gareth Taylor had initially said the "injury was not as bad as first feared", but a statement from the club on Thursday revealed that Greenwood required "a repair of her MCL".
Depending on the severity of the tear, MCL recovery can take anywhere between three and 12 months, with no timeframe given for the defender's return to action.
Forwards Vivianne Miedema and Lauren Hemp are also sidelined with long-term knee injuries, while Khadija Shaw will not return until January.
Man City were beaten 3-0 by Barcelona - as well as losing to Everton in the WSL on Sunday - in their final game before the winter break to concede top spot in their Champions League group, but had already sealed qualification to the knockout phase.
The news will come as an equally significant blow for Sarina Wiegman and England, who begin the defence of their Euros title on July 2. The 31-year-old was a key part of the Lionesses team that reached the World Cup final in 2023.
Greenwood posted on X, alongside a picture from her hospital bed: "Not the ending to 2024 that I had planned.
"I will work so hard to be back on the pitch as soon as I can, for now I'm going to take some time to myself and family who always give their time to me."
Analysis: Man City need Greenwood just as much as England
Manchester City have been hit with a severe dose of bad luck in the first half of this season. Losing Lauren Hemp was manageable, such is the dearth of attacking talent within the squad, but things have since unravelled.
Vivianne Miedema has suffered another injury setback in her fight to reach full fitness, top scorer Khadija Shaw is missing, while the absence of Greenwood has meant City have leaked five goals in the two games since her injury - losing both.
There is no underestimating how big of a blow this is to City's title chances, European hopes, and England's prospects for Euro 2025 next summer.
Without leaping to the worst-case scenario, Greenwood's full recovery could take up to nine months, though there were no details revealed about the grading of the tear.
As back-up, City have Laia Aleixandri and Alanna Kennedy - both dependable - but no replacement for Greenwood's ability to dictate from deep, or leadership to organise what is now a very young outfit. Gareth Taylor will be praying for a speedy return.
