Borussia Dortmund striker Sebastien Haller has left their pre-season training camp in Switzerland after a tumour was found in his testicle.
Dortmund said Haller "complained that he felt unwell after training on Monday" and, following medical tests, "a testicular tumour" was discovered.
Further examinations will take place "over the coming days".
Haller said on Tuesday: "Thank you all for your many messages of support and affection since yesterday's announcement. My family and I thank you. I will now focus on my recovery to come back stronger."
Dortmund sporting director Sebastian Kehl said: "This news came as a shock to Sebastien Haller and everyone else.
"The entire BVB family hopes that Sebastien makes a full recovery as soon as possible and that we can give him a hug again soon.
"We'll do everything in our power to ensure that he receives the best possible treatment."
Dortmund midfielder Jude Bellingham and Haller's former West Ham team-mate Declan Rice were among those who tweeted their support.
Bellingham said: "Stay strong and get better soon brother, we're all with you!"
Rice said: "Thoughts are with you my brother! Praying you have a speedy recovery."