The winger damaged his hamstring during the recent Johnstone's Paint Trophy success over Rotherham.
Blades boss Danny Wilson admitted in the Sheffield Star: "We brought Nathaniel here for a reason. He brought qualities to the club that we didn't have before.
"One of those was pure out and out pace so of course it's always disappointing when you lose someone.
"It means an option has gone, but there's no point in moaning and groaning about it.
"These things happen in football and you just have to get on with it."