West Ham midfielder Luis Boa Morte has suffered a suspected cruciate knee ligament injury.
The Portuguese playmaker picked up the problem during the Hammers' Asia Trophy clash with Tottenham on Wednesday.
Boa Morte lasted just 11 minutes of that contest before being stretchered from the field with his leg in a brace.
The injury occurred after the 31-year-old challenged for possession with Spurs full-back Benoit Assou-Ekotto.
West Ham medical staff tended to Boa Morte on the field, but soon reached the conclusion that he was in no fit state to continue.
The former Arsenal and Fulham man has since been whisked off to hospital for further checks, with the Hammers hoping their worst fears are not realised.
"We are still waiting," said Hammers boss Gianfranco Zola. "He has gone to the hospital for a scan but we are waiting for further news. I did not see what happened. We know it is his knee but that is all I can say."
Boa Morte featured in more than 30 games for West Ham last season as they secured a ninth-place finish in the Premier League.
To lose the 31-year-old would be a setback for Zola but the former Chelsea player is keeping his fingers crossed the injury is not serious.
"It is a big blow for me," added the Italian. "He is an important player both on the pitch and off the pitch.
"Hopefully it is not too bad as he is a vital player for us."