The Sun says the Chelsea midfield ace is awaiting the results of a scan on ligament damage and could be out until 2012.
It raises concerns about his long-term future at the top and gives new Chelsea boss Andre Villas-Boas his first major problem.
Essien, 28, collapsed in agony on Thursday after twisting his right knee.
No other player was involved in the incident, on only the second day of pre-season training under Villas-Boas at the club's Cobham HQ.
Essien had scans at a private London clinic yesterday.
The usual diagnosis for anterior cruciate ligament damage is at least six months on the sidelines. But Essien's comeback is even harder to predict.