Dortmund failed to pose any real threat aside from a 20-yard effort from substitute Karim Adeyemi in the second half, and centre-back Nico Schlotterbeck was sent off in the 73rd minute after picking up a second yellow card for a tackle on Bremen striker Justin Njinmah.
"As Borussia Dortmund, we have to perform differently, even if the team has undergone changes and has a new coach," Sahin, who was appointed in June, told a press conference.
"It would be a very cheap excuse to say it is because of the transfer window. We have worked very well in the market. Still, we are expected to play better," the coach added.
The 35-year-old boss recognised the defensive processes are already working well, as Dortmund have not conceded a goal after two games following their 2-0 home victory to Eintracht Frankfurt last week.
They will look to extend their unbeaten run at the German top flight when they host Heidenheim on September 13th before facing Club Brugge for their Champions League opener five days later.
