Sebastien Haller is expected to be out of action for "a few months" after undergoing the first stage of treatment for a testicular tumour, Borussia Dortmund sporting director Sebastian Kehl has announced.

The 28-year-old striker left Dortmund's pre-season training camp in Austria last week after complaining of feeling unwell, with subsequent tests discovering the tumour.

Haller, who joined the German club from Ajax earlier this month in a club-record deal reported to be worth €36million, confirmed on social media that the first stage of his treatment had been successful.

Providing another update on Wednesday, Dortmund chief Kehl said further treatment plans are still being discussed and no exact return date had yet been fixed for the Ivory Coast international.

"Sebastien was operated on last week. We asked for patience at the time," Kehl said at a news conference. "The therapy is currently being discussed. There are various options.

"Sebastien will be out for a few months, but once we can be more precise about that we will communicate it."

Haller excelled in the Netherlands with Ajax during a prolific 18-month spell that included 11 goals in just eight Champions League appearances in the 2021-22 season.

That included a four-goal haul in Ajax's first group-stage match against Sporting CP, as well as goals in both of their fixtures against Dortmund.

All in all, he scored in all but one of Ajax's Champions League matches – the exception being a 1-0 defeat to Besiktas in the second leg of the last 16.

Haller's layoff is a major blow to Dortmund's hopes of challenging 10-in-a-row champions Bayern Munich for the Bundesliga title this term, but they have been tipped to bring in another striker before the season begins.

"We are working on options and ideas, but there is nothing specific at the moment," Kehl said. "He will be out for two months and we have a tough schedule until November, but our squad offers options."