West Ham have confirmed defender Aaron Cresswell will be out for up to four months after suffering a knee ligament injury.
The 26-year-old left-back, who has missed just one Premier League match over the past two seasons, suffered the injury during the closing stages of Saturday's 3-0 pre-season victory over German side Karlsruher in Austria.
Cresswell was left writhing in agony on the pitch following a heavy challenge from Karlsruher midfielder Grischa Promel and immediately signalled to the bench before being replaced.
He underwent a scan on Monday to determine the full extent of damage sustained and will consult a specialist in London next week before the Hammers' medical team decide whether surgery is required.
"Whatever course of action we decide to take, Aaron faces a period of rest, followed by treatment and rehabilitation and he will be out for a period of between three and four months," said West Ham's head of medical and sports science Stijn Vandenbroucke told the club's official website
West Ham are also sweating on the fitness of midfielder Manuel Lanzini, who suffered a knee injury while on international duty with Argentina ahead of the 2016 Olympic Games in Rio.
"We are in close contact with Manuel and the AFA (Argentine Football Association), who have started a late-release replacement procedure with the IOC (International Olympic Committee)," Vandenbroucke added.
"Until he has been assessed by the IOC, he cannot be replaced, but after this process has been completed we want him to return to England to be assessed by our own medical staff as soon as possible."