The Frenchman believes that Bayern's third goal killed the atmosphere at the Emirates Stadium following Lukas Podolski's strike, and admitted that it will be difficult to turn their deficit around at the Allianz Arena.
"We kept going in the second half, but I think that the third goal really changed the tie," Giroud told UEFA.com.
"Nothing is lost yet, but we'll have to score three away from home. When you know how strong they are in defence, it won't be easy."
The second leg takes place in two weeks' time.