The 26-year-old featured for the under-21s against West Brom four days prior to the City game, and admits it was a useful run-out.
"It helped me because when you come back you need games and a bit of rhythm to be fully back," Diaby told Arsenal.com. "It was very good, and I am pleased I played in that game.
"I feel good physically. Of course I have been out for a long time so I am not at my best, but importantly I did not get any reaction [against Manchester City]. This is good for the future.
"I knew the day before the game that I was starting, so I just had to prepare for that. I had been out for a long time, so I did not expect to be on the pitch so quickly. But it doesn't matter, you just need to be prepared and see how it goes."