The blue half of Manchester secured a 4-1 win over their local rivals in September.
Despite the fierce rivalry, there is a large gap between the sides in the Premier League this season, given that City are in the title hunt, whereas United are currently 11 points adrift of a Champions League spot.
While Fernandinho believes that United were poor at the Etihad Stadium earlier in the campaign, the Brazilian is not underestimating United ahead of tonight's clash at Old Trafford.
"I was surprised how bad United were," the Daily Star quotes Fernandinho as saying. "It was the first time I'd played them after joining City.
"It's a club with a lot of history, with very good players, they were the champions, so yes I was surprised at how badly they played that day and how easily we won.
"We will go there expecting to win, but we know that in a derby a lot of things can happen. I think we will be ready for it, we'll have to play well because it's a very important game for us."
City are currently six points behind leaders Chelsea with three games in hand.