Borussia Dortmund striker Sebastien Haller left the team's pre-season training camp in Switzerland on Monday after a tumour was found in his testicle.
The Ivory Coast international joined the German club earlier this month as a replacement for Erling Haaland, who has signed for Manchester City.
"In the course of intensive medical examinations, a tumour was finally discovered in the testicles during the course of the day," Dortmund said.
"During the coming days, further examinations will take place in a specialised medical centre," they added.
Dortmund paid Ajax €31 million ($31.5 million) for the 28-year-old after he scored 47 goals in 66 appearances for the Dutch side after joining in 2021 from West Ham United.
His 21 league goals in 31 games saw him finish as top scorer in the 2021/22 Eredivisie as Ajax secured a record-extending 36th title.
Haller also etched his name into Champions League history, scoring in all six group games - Cristiano Ronaldo is the other player to have achieved that feat - and became the fastest player to record 10 goals in the competition.
"This news today came as a shock to Sebastien Haller and to all of us. The entire Borussia Dortmund family wishes Sebastien a full recovery as soon as possible and that we can hug him again soon," Dortmund's sporting director Sebastian Kehl said in a club statement.
"We will do everything in our power to ensure that he receives the best possible treatment," he added.
Dortmund's first game of the new Bundesliga season is against Bayer Leverkusen on August 6, eight days after heading to third-tier 1860 Munich in the first round of the German Cup.
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