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Dortmund played its first foreign opponent in preparations for the upcoming season, with a Goncalo Guedes brace helping Valencia to a 3-1 win in Austria.
It was overshadowed by news of Haller's diagnosis of a testicular tumour, however, ruling him out indefinitely, after the 28 year-old travelled back to Dortmund for medical tests over the weekend and missed the game.
Terzic admitted it was hard to focus on the task at hand upon his discovery of the news as the team prepared to depart for the match, but wished the Cote d'Ivoire international a speedy recovery.
Sebastien #Haller hat das #BVB-Trainingslager in Bad Ragaz krankheitsbedingt verlassen müssen und ist bereits zurück nach Dortmund gereist. Bei Untersuchungen wurde ein Hodentumor entdeckt.— Borussia Dortmund (@BVB) July 18, 2022
Gute Besserung, @HallerSeb! 🙏🙏
Weitere Infos: https://t.co/XPaNATxgDI pic.twitter.com/v6hA6MeGLV
"The week went very well until yesterday," Terzic said. "The message from Sebastien Haller shocked us. Personally, I found out about half an hour before leaving for the game.
"Accordingly, it wasn't easy for us to think about tactics, pressing and passing again. We still tried to make it as professional as possible. We didn't talk to the team about it before the game, but informed the team afterwards.
"Sebastien Haller left us yesterday and traveled to Germany for further investigations. We keep our fingers crossed for Sebastien and the entire Haller family that he will recover as soon as possible."
Initially taking over from Lucien Favre in December 2020, Terzic was reappointed as the first team coach on a permanent basis in the off-season after Marco Rose's departure.
The 39 year-old revealed he has experienced a smoother transition into the role as Dortmund boss, despite Haller's recent diagnosis.
While he believes it will take time for his BVB side to be truly ready for the season ahead, the added time to work on patterns of play has made his job less chaotic.
"The biggest difference is that when I first became head coach, I was told on a Sunday morning and we had an away game on Tuesday night," Terzic said.
"You couldn't prepare many things and decided a lot quickly and intuitively. We now have weeks of advance notice to plan this preparation and the match opponents, to shape the group and to promote the integration of the newcomers. That's a key difference.
"The preparations won't be finished next Friday either, but it will still take some time before we're where we want to be at the end and we're showing consistently good football with a lot of automatisms. It will still take a little time, but we are on the right track."