The Gunners lost 2-0 at the Emirates to defending champions Manchester City, with Laurent Koscielny seeing red after only 10 minutes.
The extra man gave City the initiative to take the game to the home side, with goals from James Milner and Edin Dzeko.
“To play 11 v 11 against City is very hard, even if you are at home, but 10 against 11 is even harder,” said Sagna.
“It is harder to play on the pitch and it is hard to have control of the ball, so we have to look forward to the next game and keep our heads up.
“City have some quality players, we have some nice players as well, but with one more on the pitch than us it makes it very difficult.”
The player believes that the manner in which his team came out to play in the second half bodes well for the second portion of the Premier League season.
“Of course we had to keep going and try and score the [Arsenal’s] first goal, and we could have come back because it has happened in the past but it didn’t happen [this time].
“We knew it would be a tough game with hard tackles and desire, and of course they wanted to win as much as we did.
“We can’t let our heads go down. First of all we need to rest because we play in two days.
“After that we just need to keep pushing because we have many games to come back and we are not far from third position, so we have to keep the heads up.”