Diame was carried off on a stretcher after 73 minutes having pulled up and fallen to the ground clutching his hamstring.
"Not good, it does not take a medical expert to know it is a serious hamstring tear when he falls and stopped the way he did," West Ham manager Allardyce told reporters.
"I would expect, in medical terms, for it to be a grade 3 (injury) and that could be anything up to eight, 10 or 12 weeks."
(Reporting by Martyn Herman, editing by Ed Osmond)
