Borussia Dortmund sporting director Sebastian Kehl has said that lessons must be learnt “very, very quickly” from a painful 5-1 Bundesliga mauling at VfB Stuttgart. “If we deliver such performances more often we will not be able to meet our goals,” Kehl warned after Sunday’s meltdown. Ambitious Dortmund have not won any silverware since the German Cup in 2021 and are five points off the top in the Bundesliga after four matchdays. They signed the likes of defender Waldemar Anton and striker Serhou Guirassy from Stuttgart, forward Maximilian Baier from Hoffenheim, and midfielder Pascal Gross fro…