West Ham defender Arthur Masuaku will be out for a minimum of six weeks after suffering a knee injury in this week's EFL Cup match against Accrington Stanley.
The 22-year-old left the pitch on a stretcher with three minutes to go during Thursday's 1-0 victory, although it was still unknown at the time about the severity of the injury.
During a press conference on Friday, Hammers' boss Slaven Bilic revealed the injury means Masuaku is to be sidelined for well over a month.
"Arthur Masuaku has damaged his ligaments and we will know better in one week when he will be back. But it will be at least six weeks," he said.
West Ham have been blighted by injuries this year and Masuaku's injury has come at a particularly bad time for Bilic as fellow left-back Aaron Cresswell is still unable to play.
"Some players can play in different positions, and you can fill gaps, but when you have two left-backs who have done the same ligaments it's not good news," he added.
However, the Croatian manager is optimistic that the number of injured players will not affect the team's performance going forward, taking the opportunity to quash recent rumours that striker Andy Carroll's recovery would take longer than previously expected.
"I saw reports that Andy Carroll had a setback but that is totally false. We are expecting him and Aaron Cresswell back after the break," Bilic explained.
"Winston Reid is back in training and Alvaro Arbeloa has joined the team. The injuries are not ideal but we will still have a good team."
West Ham have had a difficult start to the season, losing four out of five of their Premier League games so far.
The club have found it a struggle to find their form since moving away from Upton Park, having suffered a humiliating 4-2 defeat to Watford the last time they played a Premier League match at the London Stadium.
But Bilic is expecting his side's recent misfortunes will be reversed when they host Southampton this weekend.
"I am expecting us to be really good on Sunday. We had a similar position when we played Southampton last season."
Watch Man Utd v Leicester, Crolla v Linares, Aston Villa v Newcastle and Hull KR v Huddersfield on NOW TV for £6.99. No contract.