The Hammers, however, are desperate to sign another midfielder and are tempted to pay overs now for a player whose value could skyrocket if he has a successful World Cup with the Red Devils.
"Dendoncker is the centrepiece of our defence. If we let him leave it has to be for a lot of money," Anderlecht chief executive Herman van Holsbeeck said.
"We also have to find a replacement for him, even if it is a loan for six months.
"A deal can be done if the interested club puts 25 or 30million euros (£22m-26.5m) on the table for him.
"I've experienced plenty of difficult transfer windows, but this one is the trickiest of all."