New Borussia Dortmund signing Sebastien Haller will be out for at least two months after undergoing surgery for a tumour discovered in his testicle, the Bundesliga club said on Wednesday.

The 28-year-old Ivory Coast forward underwent surgery last week after discovering the tumour in a training camp, the club's sports director Sebastian Kehl told a news conference.

He said treatment plans were still being discussed and a return date had yet to be fixed. The French-born forward joined Dortmund from Dutch side Ajax Amsterdam on a four-year contract earlier this month.

"Sebastien was operated on last week. We asked for patience at the time. The therapy is currently being discussed. There are various options," Kehl said ahead of Dortmund's season start with a first round German Cup tie against 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."

The club had been banking on Haller's physical presence and goals in attack to attempt and break champions Bayern Munich's decade-long stranglehold on the Bundesliga title.

Kehl said Haller's absence will be felt but the club was preparing for a number of scenarios.

"We have been doing it for a few days. 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."