“I will have to undergo an operation to finish definitively with this tumour that keeps me away from the pitch.”
France-born Haller joined Borussia Dortmund from Ajax in July to help fill the hole left by the departure of Erling Haaland to Manchester City.
A few days after signing Haller felt pain and club doctors made the diagnosis.
“We wish you lots of strength, Seb! Stay strong!,” Borussia Dortmund reacted to Haller’s tweet. Ajax and another former club West Ham, also tweeted their support.