The 18-year-old underwent a medical at the club earlier this week after the two sides agreed a reported fee of £20m.
The move was expected to be officially announced on Thursday, prior to a delay. However, the Blues boss has said that he is still confident that the deal will be done soon.
"Bearing in mind what he has been doing in the last couple of years since starting at Anderlecht at the young age of 16 we didn't want to let this boy escape and it is a question of days or hours till this is done," he said.
"He is one of those players we couldn't afford to lose because of his potential and that's why we made the move in the market even though our team is competitive enough in that sector of the field."
The Belgium international, who has been with Anderlecht since 2006, has scored 33 goals in 73 appearances for the senior side.