The Bundesliga club, however, have no plans of selling the talented youngster and have reminded prospective suitors they hold pole position when it comes to negotiations.
"Well now the time starts again when there is some insane [transfer] talk," Leverkusen director Jonas Boldt told Kicker.
"There is no offer for Leon. We have absolutely no thought of selling him. He has a contract until 2022 - and we are on the driver's seat.
"His start at Leverkusen was not easy, he was still impressed with the hype that was being made about him in Belgium and made some mistakes.
"Today we see a boy who has learned from his behavior and is fully integrated into the team."