Goals from Jermain Defoe and Steed Malbranque either side of Didier Zokora's sending-off ended City's proud 100% home record, stretching back 10 matches.
Speaking through an interpreter, he said: "It is a positive result and sets us up for the rest of the season.
"To come away with a 2-0 victory will stand us in good stead and help the mind-set of the players.
"Given the circumstances, it has to be the best performance since I took charge.
"We were away from home against a team like Manchester City, who are high in the league and on a good run of form at home.
"Not only that, we were a man down from the 20th minute yet still managed to get through. We got our just rewards."
Zokora was dismissed for lunging at Elano, but Ramos said: "It wasn't a sending-off, not in a game like this.
"A cup tie generates a heated atmosphere, and the referee has to be a bit more flexible. It was unfair."