The two clubs are yet to agree on a fee for the 18-year-old as the Belgian outfit are believed to want an initial fee of £13m with add-ons, while Chelsea want to pay a smaller down payment and more appearance-based bonuses.
"There is no agreement at the moment," said Van Holsbeeck.
"The dream of our club chairman [Roger Vanden Stock] is to keep Lukaku to play with our new player, Milan Jovanovic, in the team. But we have to be realistic.
"There are ongoing negotiations and Chelsea know what we want. Imagine that, one day, Roman Abramovich is in a good mood and he offers that amount and this can be fixed tomorrow."
Lukaku scored in Anderlecht's 3-1 win over KV Mechelen in the Belgian Jupiler League on Friday night.