Borussia Dortmund has a new coach and hopes that a flurry of new signings will help turnaround an underwhelming season so far. Niko Kovač started work as coach on Sunday, taking over from unbeaten interim coach Mike Tullberg, who stepped up from the club’s under-19s for three games after Nuri Sahin was fired. Sahin, a club favorite from his playing days, was let go as coach after starting the year with four defeats.