Haller who hails from Ivory Coast complained that he felt unwell after training on Monday morning.
Following a series of thorough medical examinations conducted during the course of the day, a testicular tumour was ultimately discovered. Further examinations will take place in a specialist medical facility over the coming days.
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“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,” BVB sporting director Sebastian Kehl said.
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Borussia Dortmund requests that the privacy of the Haller and his family is respected and that no inquiries are made. As soon as we have further information, we will consult with the player and inform you.