In his first taste of the Manchester derby, the Brazilian midfielder helped City to a more than comfortable 4-1 hammering of the Red Devils.
And the £30million summer buy from Shakhtar Donetsk admits he is expecting a much harder game on Tuesday as they travel to Old Trafford - a game you can see live on Sky Sports.
"It was the first time I'd played them after joining City," he said.
"United are a club with history, with very good players and they were the champions. So I was surprised at how badly they played, how easily we won.
"The situation is different now, because we're fighting for the title and they're not. They're just fighting for honour now. But a derby is the same whichever country you are in - each team wants to win.
"We'll go there expecting to win, but we know in a derby a lot of things can happen. We'll be ready for it and have to play well because it's a very important game for us."
Fernandinho feels it is City's midfield options that gives them the edge over United.
"The midfielders are doing well right now and I think it's the right moment for us to be in good form," he said.
"It's important because our strikers are not scoring at the moment. But the midfield is keeping up the level of performance by scoring goals as well.
"But we can't go thinking that their midfielders won't score against us, we need to be careful.
"We hope our strikers can get back to the way they were at the start of the season. But until then, we'll carry on trying to score the goals for them."
Watch Manchester United v Manchester City live on Sky Sports 1HD from 7pm