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However, beforethe coronavirus-enforced break in the season, the Bundesliga side's head coach Peter Bosz insisted that a prized asset would not be sold for less than €100m(£90m/$111m, while talking up his rapid rise to prominence in Germany.
Chelsea have already spent a combined total of €93m(£84m/$110m)to bring in Hakim Ziyech and Timo Werner from Ajax and RB Leipzig respectively, and Voller has now confirmed that they will have to fork out a club-record fee to lure Havertz away from his current surroundings.
Asked if Leverkusen have already agreed to sell Havertz, the 60-year-old told Gazzetta dello Sport:"No, but with his talent, it means he's on the list of all the best clubs.
"We know of the interest of a couple of clubs, especially Chelsea, but for us, the situation is quite simple; he has a two-year contract, if anyone wants him on our terms, fine, otherwise he stays here another year and we are happy.
"It won't be easy, but for an artist like him there is no Covid discount."
"Of course he will play in the Europa League quarter-final, also because he wants to," he said. "I am a bit old-fashioned: for me a season is over when it all ends.
"Nobody leaves first. Now, with Covid-19, there have been transfers before the end of the season, but not from us."
Voller added on Havertz's talent: "For me, he's the best I've seen at Leverkusen, and I've been here for a lifetime. We had Emerson, Michael Ballack, Ze Roberto, Toni Kroos, but he's on top.
"He is very young, he can play five or six roles, he has been here for 10 years. When he was 17, I asked the first-team coach to let him train with the greats during the national team break.