Highly-rated attacker Morrison joined West Ham in January 2012 after failing to establish himself in Sir Alex Ferguson's plans at United.
The 20-year-old is now starting to fulfil his potential at Upton Park and was rewarded for his fine form this season with a call-up to the England Under-21 squad.
Allardyce admits he is unsure if United have a buy-back option in the deal when they sold Morrison and concedes the Hammers would struggle to resist any big-money offers for the skilful midfielder.
"I don't know if there is a buy-back clause and what that figure might be if it even exists," Allardyce is quoted as saying in the Sunday People
"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 £10million.
"Then there was Phil Jones when I was at Blackburn - that's what you are up against."