​Borussia Dortmund are not expecting injured centre back Nico Schlotterbeck to return until late September at the earliest, managing director Lars Ricken said as the squad gathered for pre-season training on Tuesday.Schlotterbeck suffered an ankle injury at the World Cup playing for Germany.AdvertisementAdvertisementAdvertisement”It will take a few more weeks. We have to say that honestly. Whether that will be before or after the first international break, we’ll see how the healing progresses,” Ricken told reporters.The first international break of the new season is at the end of September.Dortmund captain Emre Can will have to wait even longer for a comeback after his anterior cruciate ligament rupture.It leaves the Black and Yellows, a distant second in the Bundesliga behind Bayern Munich last term, short of numbers.New sporting director Ole Book had hoped to be able to present two established attacking players at the start of training.AdvertisementAdvertisementAdvertisementBut so far, the club have signed prospects Kaua Prates, 18, from Brazil, Justin Lerma, 18, from Ecuador and centre back Joane Gadou, 19, from RB Salzburg.”We are taking the time so that in the end we sign the players who will also improve us,” Ricken said.According to multiple media reports, Dortmund are interested in Konstantinos Karetsas, also 18, from Genk in Belgium. The attacking talent could replace Julian Brandt, who has not yet found a new club after this Dortmund deal ended.Finalizing the planned sale of Karim Adeyemi to Barcelona could accelerate Dortmund’s transfer activities.Niko Kovač’s Dortmund start their competitive season with the Franz Beckenbauer Super Cup against double winners Bayern on August 22. The Bundesliga begins a week later.