When asked if Ajax had received any bids for the Holland striker, he told Sky Sports News: "No, and I hope that we don't receive them."
He added: "It's difficult to keep them. I think we have a good youth system, which is very important to Ajax, but it's been like this for the last 25 years.
"It's important to keep players for as long as possible, maybe with money, maybe with other promises.
"If they go after playing for five years in the first team of Ajax, that is OK. But now they are going at the age of 18 or 19 and that is too quick."