The midfielder has emerged as a target for Liverpool and Manchester City following an impressive performance against Roberto Mancini's side in the Champions League.
But De Boer, brother of Ajax manager Frank, has urged Eriksen to spend at least one more season with the Amsterdam-based club as the player continues to stall on a new contract.
"Eriksen is only 20, but already seems older than he is. He has so much talent and of course it is difficult to keep him," he said.
"I think he is a player for Madrid or Barcelona but I think one more year at Ajax wouldn't hurt him. Although that's difficult - I can't predict next year he will be at us.
"It would be great to have him in the team but if a team comes along and puts a big bag of money it would be impossible for Ajax to keep him."