Bosz said he “does not expect” either to return after the international break before adding: “in soccer, it can always fail in the end”.
Havertz, a 21-year-old attacking midfielder who made his international debut two years ago, is one of the most exciting young talents in the German game.
“Quite frankly, discussions have been going on for a long time and I assume that there is a better chance of Kai leaving than of him staying,” said the Dutch coach whose team narrowly missed qualifying for the Champions League by finishing fifth in the Bundesliga.
Before the coronavirus crisis, Leverkusen were asking 100 million euros (120 million dollars) for Havertz.
Chelsea have already acquired another Bundesliga star, Leipzig striker Timo Werner, for an estimated 53 million euros.
Striker Volland, who is 28, has 10 Germany caps and has scored 44 Bundesliga goals in four seasons in Leverkusen.
“If you ask me if I expect him to come back, my answer is clear: ‘no!’ ” said Bosz.