The 28-year-old Ivorian footballer has been undergoing chemotherapy after being diagnosed with testicular cancer in July.
He wrote on Twitter: ‘As planned from the beginning, different possibilities were considered following the chemo.
‘I tell you that the fight is not over for me. I will have to undergo an operation to permanently end this tumour which keeps me away from the pitch.’
Comme prévu depuis le début, différentes possibilités étaient envisagées suite aux chimio. Je vous annonce que le combat n’est pas terminé pour moi. Je vais devoir subir une opération pour en finir définitivement avec cette tumeur qui m’éloigne des terrains.
Merci à tous 💪🏽🙌🏽 pic.twitter.com/sFijTijLXu
— Sébastien Haller (@HallerSeb) November 16, 2022
Haller spent 18 months with West Ham after joining in 2019 for a then club-record £45million from Eintracht Frankfurt.
He moved to Ajax in January last year and signed for Dortmund this summer.
The Ivorian international has scored four goals in 15 appearances for Ivory Coast.
Haller had been training with former club Ajax as part of his recovery as he was rehabilitating in Holland.
After feeling unwell during Dortmund’s pre-season tour of Switzerland, he underwent tests which revealed a malignant tumour.
Haller is yet to make his debut for the Bundesliga giants and has previously opened up about the treatment he has undergone.
Speaking to UEFA last month, he revealed he has spent ‘five days at a time at the hospital, where I am hooked up 24/7’ and cannot get out of bed while the treatment is injected. He then has a two-week rest period.
‘That’s one phase, and I have to do that four times. Four phases of chemotherapy lasting roughly three weeks each,’ he added.
‘After that, depending on how my cancer is progressing and how it is spreading, I may be forced to undergo surgery.
‘A lot of people are asking me when I will be back, but there’s a lot to take into account so it’s hard to give them a straight answer.