DORTMUND, Germany: Borussia Dortmund striker Sébastien Haller has undergone a successful operation on a testicular tumor and will be out for at least “a couple of months,” sporting director Sebastian Kehl said Wednesday. Haller signed for Dortmund from Ajax on July 6 for 31 million euros ($31.5 million) but had to leave a pre-season training camp 12 days later for treatment after the tumor was found. Kehl said Haller’s operation was successful but added the next steps in his treatment were still not fully clear.