Norwich defender Timm Klose could be out for up to a year if the worst-case scenario with his knee injury is realised.
Klose suffered the problem in the first half of the midweek Carabao Cup defeat to Crawley and scans have shown posterior cruciate ligament damage which could need surgery to repair.
It is a blow for the Swiss centre-back, who was making his first appearance of the season, and the full prognosis will be revealed after a second scan next week.
Boss Daniel Farke told the club's official website: "Timm Klose has had a scan but he will need another scan next week.
"He has ruptured his PCL [posterior cruciate ligament] and may need surgery. It's difficult to judge at the moment, and it depends a bit on the result of the rescan. We are concerned with his meniscus.
"The best-case scenario is he will be out for four months. There's a danger there is something wrong with the meniscus, so the worst-case scenario from the doctors is nine to 12 months.
"Timm was unbelievably disappointed and it's important to let him be disappointed, 2019 hasn't been his year, he's had quite a few injuries.
"The opponent slipped and it's tough to take. He worked so hard to come back, and we will support him 100 per cent with his recovery.
"He will come back stronger than before, and he's a brilliant footballer and a brilliant lad. We will give him all the time he needs.
It deepens Farke's issues at the back as Grant Hanley has been ruled out of the trip to West Ham after suffering a groin injury in training.
However, Christoph Zimmermann is fit to face the Hammers after a recent hip problem.
"Christoph Zimmermann is available," Farke added. "He got a hit against his back and hip and was sore but he trained today. He's been out for a long time but he's available.
"Unfortunately Grant Hanley is out for tomorrow. He injured himself this morning and overstretched his groin. He needs a scan today and I'm not sure he can travel with Scotland [for international duty]."