The Gunners had a lot of possession during the game, but were unable to create too many clear cut chances.
Asked about the team's lack of aggression, he told reporters: "I think fatigue had a say as well. We didn't look as sharp on the ball. We didn't have as much purpose on the ball to create.
"Obviously, David was giving us something different when we had to attack the low block with his ability to attack the line and provoke, always threatening in behind. We lacked that a little bit in the second half.
"Still, credit to them, they are really organised when they do that. It is really difficult to create the chances. We created a really good one with Hector again and we didn't convert. That's the difference at the end, in the box, what we are able to do.
"Certainly, it's an aspect that we have to improve."