Former West Ham forward Sebastien Haller has been diagnosed with a testicular tumour.
The 28-year-old underwent medical examinations on Monday after feeling unwell following training with Borussia Dortmund, the club said.
A testicular tumour was discovered during those examinations, with further tests to take place in a specialist medical facility over the coming days.
Borussia Dortmund’s sporting director Sebastian Kehl said: “This news today 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.”