The Argentine striker scored twice in the Citizens' 3-1 away win in the Premier League on Saturday
"I did not think we deserved to concede," BBC Sport quotes Richards as saying. "We defended well today, and Sergio Aguero was outstanding.
"It was tough at times. They did not play with a recognised striker, it was tough to pick a man out at times. We pressed well, and did well at the end.
"It is a nice boost for us [Manchester] United drawing, but Arsenal and Chelsea won. I think games like this will be the difference."
City are currently fourth in the league table with 16 points from eight matches.