Havertz left Bayer this past summer and after a promising start in England is now struggling to handle the demands of the Premier League.
The Germany international was handed a start by manager Frank Lampard for their third round FA Cup tie with Morecambe and repaid his manager with a goal in the 4-0 triumph.
However, there remains doubts about Havertz and whether his game suits Premier League football. Watching on is Bosz, who had Havertz playing the best football of his career last season at Bayer before overseeing his sale to Chelsea last summer.
"But it's very good that he defends him in the press."
The Dutchman, though, insists Havertz needs to look inward and prove himself - particularly given the money Chelsea paid Bayer for his transfer.
He continued: “In the end you just have to deliver, you have to score goals and prepare.
"As a 100 million transfer you have to play and he doesn't."
This season, the 21 year-old Havertz has played 22 games for Chelsea, scoring five goals and producing six assists.
- updated January 11
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!