New Chelsea boss Thomas Tuchel is hoping Kai Havertz can toughen up to meet the demands of the Premier League.
Havertz, 21, joined Chelsea from Bayer Leverkusen in a £71m deal last summer but has struggled in his 17 league appearances for the Blues, scoring just once and notching three assists.
Tuchel said: “What I feel is he’s a very intelligent, friendly, open guy, with a lot of potential.
“He seems to be the guy who needs to be pushed. We will not stop doing it.
“This league is perfect because he cannot rest. He needs to be challenged physically – and that it’s not enough to rely on his endless talent.
“For me, the question is can he show his teeth when it gets tough?”
He believes there is almost no limit to Kai Havertz’s potential.
Tuchel said: “Kai’s potential is almost endless.
“He made a very brave choice. You come from Leverkusen to Chelsea, the culture cannot be more different.
“You come from a club where it’s okay to be second or third best, to a club where they talk about trophies.
“It means Kai steps not only to a different country, not only to a different club, not only to different teammates – but a different culture.
“I love it because he is out far from his comfort zone.”