Ivory Coast’s Haller 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 for the 28-year-old after he scored 47 goals in 65 appearances for the Dutch side.

“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 Aug 6, eight days after heading to third-tier 1860 Munich in the first round of the German Cup.