The Belgian has made a real impression since arriving at Manchester City on loan from Marseille.
Keegan would love to push through a permanent deal, as van Buyten has hinted at recently, but the former England chief believes the Belgian is destined for a bigger stage.
"He is terrific player and I don't think we will keep him at the end of the season as I feel he will go to a big club at the end of the season," said Keegan after van Buyten performed superbly against Chelsea.
"The deal we have is not tight and other teams are sure to nip in.
"Anyone who has watched him today can't fail to have been impressed."
