Chelsea are chasing the midfielder but face competition from the likes of Manchester United and Bayern Munich – with Liverpool also linked.
His form in the Bundesliga this season has meant that his future is the source of great speculation but, with his contract set to expire in 2022, Leverkusen has the ability to play hardball with any interested parties.
Indeed, reports have suggested that the Blues would have to pay around £75m to land the 21-year-old.
“Many clubs are interested in him,” he said. “I see the player’s wish to take the next step sometime in the near future. “You have to respect that as much as the fact that he has a contract in until 2022.”
“Kai has been with us for ten years, he is a player who is fully integrated and devoted to the club, but he wants to take a big step,” he said.
“All the great clubs in Europe are after him and for us, in the end, what the player wants is what is important.
“We are open to see what will happen in the future, nobody knows.
“At the moment he is focussed on the season with us. He has been injured the last two games but we hope to have him for the last few fixtures.”