The reigning Barclays Premier League champions had not won at the home of the Gunners since 1975, but eased to victory thanks to goals from James Milner and Edin Dzeko.
Arsenal's task of beating the champions, who remain seven points adrift of leaders Manchester United, was made harder when defender Laurent Koscielny was sent off after just 10 minutes.
France international Sagna admits it was always going to be difficult once referee Mike Dean had brandished an early red card, even though Dzeko missed the resulting penalty.
"To play 11 v 11 against City is very hard, even if you are at home, but 10 against 11 is even harder," Sagna told Arsenal Player.
"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.
"It is always nice to see a goalkeeper saving a penalty but it is difficult to play.
"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."
- Sports & Recreation
- Edin Dzeko
- James Milner
- Laurent Koscielny
- Manchester United