The Italian centre-back was injured in his side's 3-2 win over Liverpool on Sunday.
Angelo Ogbonna suffered an anterior cruciate ligament (ACL) injury during the Hammers'victory
The former Juventus defender was subbed off in the first half, with additional scans and tests being done on Monday. West Ham's team doctor Richard Collinge says the 33-year-old has already started his recovery process.
"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 been a key player for the Hammers in the current campaign. The defender has started all of the Hammers' league matches this season as they sit third in the Premier League with 23 points.
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