The Blues retain an interest in Sancho but, as ESPN reported in April, they fear the 20-year-old favours a move to Old Trafford in a deal which could rise to as much as £120 million.
Chelsea head coach Frank Lampard is keen to overhaul his squad this summer with the club closing in on the £49m signing of Timo Werner from RB Leipzig in addition to the £38m capture of Hakim Ziyech from Ajax.
They are in the market for another attacking player -- while also pursuing Leicester City left-back Ben Chilwell -- and are keeping tabs on Sancho while recognising a need to identify alternative candidates.
Chelsea are one of several clubs to have been offered the chance to sign Philippe Coutinho from Barcelona, either as a loan or on a permanent deal, but have so far proved reluctant to match the £9m loan fee and £250,000-a-week wage.
A transfer would cost in excess of £70m, a similar fee to Havertz who is seven years younger and viewed as one of the most exciting talents in Europe.
Lampard views Havertz's versatility as a key factor given he can play as a No. 10, a central striker or out wide. His contract expires in 2022 but Leverkusen are increasingly resigned to losing him this summer.
Bayer sporting director Simon Rolfes said: "We don't know yet what will happen to Kai. We know that all the great clubs from Germany and Europe are after him and that he has the ability to play anywhere."