The Hammers signed the 20-year-old attacking midfielder from the Red Devils in the summer of 2012 for a fee of around £650,000, and the youngster has impressed so far this season.
"I don't know if there is a buy-back clause and what that figure might be if it even exists," Allardyce told reporters. "But my real big worry is that Manchester United will come back to buy him.
"Look at what happened when Chris Smalling played a handful of games for Fulham - United jumped in with £10 million. Then there was Phil Jones when I was at Blackburn – that's what you are up against."