Borussia Dortmund sporting director Sebastian Kehl has confirmed new striker Sebastien Haller has been ruled out of action for 'a few months' after he began treatment for a testicular tumour.
The former West Ham forward, 28, complained about feeling unwell following training last Monday and immediate medical examinations detected the tumour.
The Ivory Coast international joined Dortmund from Ajax earlier this month for a reported £27million, making him the most expensive striker in the German club's history.
Haller's Dortmund debut has been delayed as a result of the treatment following the shock diagnosis. An exact timeframe has not been put on his return to action.
'Sebastien was operated on last week. We asked for patience at the time. The therapy is currently being discussed,' Kehl told reporters ahead of the German Cup first-round clash with 1860 Munich on Friday.
'Sebastien will be out for a few months but once we can be more precise about that we will communicate it.'
Haller, who has effectively replaced Erling Haaland after the Norwegian star left Dortmund to join Manchester City, completed the first step of his treatment last week.
Posting a picture from his hospital bed on Thursday, a smiling Haller tweeted: 'Hello everyone, I wanted to let you know that Step 1 has been completed!
'I would like to thank the @BVB (Dortmund) and the medical team who were exceptional with me.
'A big thank you also to all the nursing staff of the hospital for their support - benevolence.'
Haller added prayer and flexed biceps showing strength emojis to the tweet.
The Ivorian signed a four-year deal with Dortmund after scoring 32 goals in 36 appearances in all competitions for Ajax last season.
Haller spent one-and-a-half seasons at West Ham, making over 50 appearances, before the Hammers sold him to Ajax for £20.25m in January 2021.
Dortmund quote tweeted Haller's message and said: 'Always a smile on his face...get well soon Seb!'
West Ham replied to their former player's tweet: 'We're all with you, Seb!', adding a heart emoji.