Dortmund controlled possession but had to wait until the 21st minute for their first clear-cut chance, as Haaland's header called Yann Sommer into action.
Hertha took the initiative from the kick-off whereas Bremen focused on their defence.
World no. 9 Stephen Maguire only managed to draw twice and was beaten by Dale 3-1.
"Signing for this special club was never in doubt," the striker told the Arsenal website, adding it was "thanks to our fans, my teammates, my family and everybody at this club that I feel like I belong here."
The hero of the night became the home team's Moldavian striker Ion Nicolaescu who tied the game five minutes before the end of regulation on his debut and then put Dunajska Streda ahead 4-3 six minutes into overtime with a quick and un-saveable shot from the edge of the box.
Even though Hertha kept their efforts to a minimum, they further extended their lead on 63 minutes as Jhon Cordoba assisted Matheus Cunha, whose low shot made it 3-0.
Flamengo remain second in the group with six points from three matches but with a negative goal difference.
Back in May, several countries followed the successful example of playing football matches behind closed doors in accordance with strict COVID-19 hygiene regulations.
Teda took the lead in the 70th minute when Sandro Lima donated a powerful header on Frank Acheampong's cross. However, six minutes later, Teda's advantage was terminated as their newly transfered midfielder Akhmedov was substituted with injury.
"It's not true that I'm not counting on him," insisted Koeman, who believes a loan move is likely.
Dortmund CEO Hans-Joachim Watzke has demanded that everybody obey the rules and "diligently take the great chance we have been given in sports."