It was Giroud's flick from a cross that got Chelsea going in the game, giving them a lead to take to half time.
The Blues eventually ran away 3-1 winners in a one sided contest. But Giroud believes United keeper David De Gea should have saved his shot.
He told the club's website: "I try to stay in front of my defender and Azpi had time to cross the ball well, and I like to take it with the outside of the left.
"I was a bit lucky in the end because it was a bit on David De Gea, it was so close.
"But it went quick and I think that is why he didn't save it."