Dortmund confirmed via their official social media handles, that the former Ajax star and club's new signing would exit the club's Swiss pre-season training camp after suffering from illness.
Dortmund star forced out of pre-season camp due to 'Testicular tumour'
Borussia Dortmund's summer signing Sebastien Haller, has been forced to leave the club's pre-season camp after he was diagnosed with a 'testicular tumour'.
A statement on the club website read: "Borussia Dortmund striker Sebastien Haller had to leave the BVB training camp in Bad Ragaz, Switzerland, early due to illness and has already travelled back to Dortmund.
"The 28-year-old international from the Ivory Coast complained of being unwell after training on Monday morning.
"In the course of intensive medical examinations, a tumour was finally discovered in the testicles during the course of the day.
"During the coming days, further examinations will take place in a specialised medical centre."
BVB sporting director Sebastian Kehl said: "This news today came as a shock to Sebastien Haller and to all of us.
"The entire BVB family wishes Sebastien a full recovery as soon as possible and that we can hug him again soon.
"We will do everything in our power to ensure that he receives the best possible treatment."
Borussia Dortmund suffer early Sebastien Haller blow
Sebastien Haller just arrived in a £28million deal from Ajax earlier this month as a potential replacement for Erling Haaland who sealed a switch to Manchester City.
The towering striker had a prolific 18-month spell at Ajax since arriving in January 2021.
The Ivory Coast international scored 13 goals in 23 matches during his first season for the Dutch giants under former manager and current Manchester United boss Erik Ten Hag.
Haller even improved his tally the following season by becoming the top scorer in the Eredivisie last term after netting 21 times, as well as scoring 11 times in eight Champions League matches.
The 28-year-old will now have to wait to make his debut for the Yellow and Black outfit following the latest developments.
Meanwhile, Dortmund has asked for the privacy of the player and his family to be respected for the time being.
"Borussia Dortmund asks that the privacy of the player and his family be respected and that no questions be asked.
"As soon as we have further information, we will inform you in consultation with the player." the statement added.