The 20-year-old midfielder has burst onto the stage this season with some excellent performances culminating in his wonder goal against Spurs last time out but that has led to speculation claiming there is a clause in his current deal which could see him return to United.
Allardyce says he is unsure whether there is a release clause but he is still worried that United could just nip in and snap Morrison up if they wanted to.
"I don't know if there is a buy-back clause and what that figure might be if it even exists," Allardyce said.
"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."
West Ham last week