Haller was not feeling well on Monday after training in Switzerland. Borussia Dortmund initially reported that the attacker was ill. Examination at a hospital showed that he has a tumor near a testicle.
Haller has returned to Germany and is under further investigation there.
“This news came as a shock to Sébastien Haller and to all of us today. The whole Borussia family wishes Sébastien to be completely healthy as soon as possible and that we can embrace him again soon. We will do everything we can to ensure that he receives the best possible treatment,” said sporting director Sebastian Kehl.
The club wants the striker’s privacy to be respected and calls on the media not to ask any further questions.