The Swiss international has been immense for the Hammers this season in midfield, often praised for his excessive work-rate.
Behrami left the field on a stretcher, in obvious agony, after catching his studs in the ground as he went for the ball just before half-time.
An initial MRI scan has thankfully shown there is no break or fracture although the swelling is currently too bad to allow for a better diagnosis.
Gianfranco Zola told the official site, "Hopefully it is not going to be too bad as the boy is a really important player for us - on the pitch and off pitch as well. I think his studs got caught and he fell on his knee. He was in pain so he needed some oxygen."