Chelsea boss Thomas Tuchel has declared Kai Havertz has "endless potential".
The struggling summer arrival is the second-most expensive signing of the Roma Abramovich era.
Tuchel said: "His potential is almost endless and it is the challenge for him — and I think he made a very, very brave choice.
"I am a huge fan of Bayer Leverkusen. But you come from Leverkusen to Chelsea, the culture cannot be any more different.
"You come from a club where it is maybe OK to be second best or third best, where you compete to reach the top four. Then you go to a club where they talk about trophies.
"And if a club talks about winning trophies and titles so openly, there is a certain atmosphere. So it means that Kai steps not only to a different country, not only to a different club, not only to different team-mates — which in itself is all complicated and has different challenges.
"But, no, he also steps into a different culture. In a dramatic way.
"And I love it because he is out far, far from his comfort zone."
And Tuchel has a plan for Havertz, 21. The Blues head coach said: "What I feel and hear from him is that he is a very intelligent guy, very friendly guy, very open guy, with a lot of potential — and we will push him.
"And he seems to be like the guy who needs to be pushed. We will not stop doing it.
"I think for him this league is perfect because he cannot rest.
"He has so much potential that he needs to be challenged physically also, that it is not enough that he can rely on his endless talent.
"You know in the end, for me the question is — and you see it in me — does he have this?"
