“I will have to have an operation to put an end to this tumor that keeps me off the field for good.”
Frenchman Haller joined Borussia Dortmund from Ajax in July to help fill the void left by Erling Haaland’s move to Manchester City.
A few days after signing, Haller felt pain and was diagnosed by club doctors.
“We wish you strength, Seb! Stay strong!” Borussia Dortmund responded to Haller’s tweet. Ajax and former club West Ham also tweeted their support.