The 26-year-old striker has scored 16 goals in the Eredivisie since joining from J-League side Ventforet Kofu in January of 2012.
"There have been a couple of agents who contacted us," Van Leeuwen told Voetbal International. "They told us they were working for English clubs.
"I have told them they could send in a mail or fax on the writing paper of that specific club. After it I haven't heard anything more. It's logical that clubs are interested.
"Is he for sale? Everyone is, but we don't want to see him going. However, the market is still open for another month and we also have other options to fill in."
Havenaar's parents moved to Hiroshima when his father joined Mazda FC from Den Haag in 1986.