The Senegal man joined the Hammers on a free transfer from Wigan in the summer, and has won plaudits for his dynamic box-to-box play in the heart of the Irons' midfield.
His form has also attracted attention from West Ham's Premier League rivals, with Goal.com exclusively revealing that Arsenal are interested in the ex-Latics man.
“It’s not looking good,” the West Ham boss told reporters after the 3-2 loss to Liverpool.
“It doesn’t take a medical expert to know it’s a serious hamstring tear. I’d expect it to be a grade three, which can be anything up to 12 weeks.
“He’s about the start and build-up of our creative attacking game, getting through the midfield barrier the opposing team put up and driving at their back four. He drives them back into their own box.
|THE BUBBLE BURSTS|
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“We will have to be a bit more dogged and resilient now.”
After their impressive win over Chelsea last Saturday, the Irons came unstuck against Liverpool at Upton Park on Sunday.
Goals from former West Ham players Glen Johnson and Joe Cole helped condemn Allardyce’s men to defeat, and the Hammers boss says the result took him by surprise.
“It was a weird day,” he added. “You wouldn’t expect five goals between two sides who defend well, and you wouldn’t expect two former West Ham players to score.
“We’ve got to take it on the chin. We played as well as we could. It’s disappointing to lose but it’s even more disappointing when your most creative player leaves the field with a serious injury.
“We’ve got to try and continue our good form and our good results. We’ve been going through a good period but the biggest and most disappointing thing is the injuries.”