However, Voller has insisted that Leverkusen will not sell the 21-year-old on the cheap despite the financial impact of the coronavirus outbreak.
"Havertz? We know that Chelsea are strongly interested in him - but we'll not sell any player before the end of the Europa League," Voller told Gazzetta dello Sport. "If Chelsea want to sign Kai, they must pay what we ask. No lower price because of Covid."
The attacker has been in impressive form at club level during the 2019-20 campaign, scoring 17 times and contributing nine assists in 44 appearances in all competitions.
Leverkusen will face Inter Milan in the quarter-finals of the Europa League on Monday night.