Sebastien Haller will undergo chemotherapy treatment after his testicular tumour was revealed to be malignant.
The forward, who joined Borussia Dortmund from Ajax this close-season, was diagnosed with a tumour earlier this month after complaining of feeling unwell at BVB's training camp in Austria.
Haller last week underwent surgery in the first stage of his treatment.
Dortmund sporting director Sebastian Kehl had confirmed Haller would be missing "for a few months" but added further treatment plans were still being discussed.
And a Dortmund statement on Saturday confirmed Haller's diagnosis and the need for chemotherapy.
Haller kämpft gegen bösartigen Tumor— Borussia Dortmund (@BVB) July 30, 2022
Sebastien #Haller wird dem #BVB mehrere Monate lang fehlen. Der histologische Befund ergab einen bösartigen Hodentumor. Wir wünschen Dir weiterhin viel Kraft, @HallerSeb !
Zur Meldung: https://t.co/8WAUXVVJ55 pic.twitter.com/dqSWhkCGZO
The statement, which updated that Haller would be absent for "several months", said: "We ask for the understanding of the media and fans that, beyond the information mentioned, we will not disclose any medical details regarding the treatment of Sebastien Haller today or during the coming months."
Kehl added: "Sebastien will now receive the best possible treatment. The chances of recovery are very good. We wish him and his family a lot of strength and optimism and are with him with our thoughts at this difficult time."