West Ham defender Kurt Zouma is set for a spell on the sidelines after suffering a hamstring injury in the 3-2 league win over Chelsea on Saturday.
The 27-year-old France international had a scan on Monday, but the club have not set a timeframe on his recovery.
Fellow centre-back Angelo Ogbonna is set to miss the rest of the season with a knee injury picked up in November.
The shortage of defensive options may prompt boss David Moyes to move for reinforcements in January.
Zouma has started 11 of West Ham's 15 Premier League games this season since his £29.8m summer move from Chelsea, playing a key role in the Hammers' surge to fourth place in the table.
With Ogbonna out and veteran Winston Reid having left the club in September, Moyes has only Issa Diop and Craig Dawson as fit specialist centre-backs.
Full-back Ben Johnson has a "low-grade" hamstring tear from the Chelsea win but the 21-year-old is set to return sooner than Zouma.