West Ham United have confirmed Angelo Ogbonna sustained an anterior cruciate ligament injury during Sunday's Premier League win over Liverpool, although a timeframe on his recovery remains unclear.
A scan on Monday revealed damage to his right knee, and West Ham said the Italy defender has started his recovery programme at Rush Green.
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.
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"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."
Recovery times can vary for ACL injuries, although Ogbonna is set for a lengthy spell on the sidelines and could miss the remainder of the campaign.
Liverpool defender Virgil van Dijk missed the rest of the 2020-21 season when injuring his ACL last October and was back playing nine months later.
Meanwhile, Leicester's James Justin sustained ACL damage in February and is yet to return to action - Brendan Rodgers hopes to have him back this month after the player resumed light training.
Ogbonna started the 3-2 win over Liverpool and played a part in the opening goal at London Stadium when putting pressure on goalkeeper Alisson Becker, who got a touch on the cross to ensure it went down as an own goal.
Later in the first half, Ogbonna went down and received treatment for the knee injury, though at the time he played on for a few more minutes.
During that period back on the pitch, Ogbonna was left bleeding after Diogo Jota barged into him, and he was eventually brought off for Craig Dawson in the 22nd minute before going down the tunnel for further treatment.