West Ham defender Angelo Ogbonna suffered an anterior cruciate ligament (ACL) injury in the Premier League win over Liverpool on Sunday.
The former Juventus centre-back did the damage in the first half of a 3-2 victory for David Moyes' high-flying side at London Stadium.
Ogbonna underwent a scan on Monday and has already started out on the road to recovery, with the Hammers waiting to discover how long he faces on the sidelines.
The London club's head of medical, Richard Collinge, said: “After being substituted following a separate incident that led to him suffering a cut above his eye, Angelo felt some discomfort in his right knee so we scanned him to ascertain the extent of the injury.
"That scan has shown up some damage to the anterior cruciate ligament and we have begun his rehabilitation immediately. Angelo will see another specialist this week and we will then have a clearer picture around his timeline of recovery."
Ogbonna has formed an imposing central defensive pairing with Kurt Zouma following the France international's move from Chelsea, after enjoying a strong 2020-21 campaign.
The Hammers are third in the top flight, three points behind leaders Chelsea, and are on course to qualify for the Europa League round of 16.