Manuel Pellegrini's men were humbled by the Latics for the second time in as many seasons as they were beaten 2-1 at home in the quarter-final, a result which left the Frenchman lost for words.
With a trip to Camp Nou for the second leg of their Champions League last-16 encounter next up, however, Nasri is keen to focus on turning around a 2-0 deficit.
He told reporters: "We feel hung-over. It was a bad performance against Wigan and we didn't play well. It's the FA Cup, [the draw] gave it to us on a plate and we refused to take it.
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"Thank God we play a big game on Wednesday. It's the best way to forget this game and be focused on the next one.
"It's going to be a big game. We're going to think about Barcelona, we need to be positive and get a result there. A game like this, you cannot not be focused from the start. It's an amazing game and we can do something fantastic if we change the result but we have to be focused and take [the Wigan] game as a lesson."
Though Barca are the bookmakers' favourites to seal their progression on home soil, Gerardo Martino's men have lost two of their last three matches in La Liga and Nasri believes that City can approach the game with freedom as they have "nothing to lose".
"We have to do everything from the start," he added. "It's a special feeling to play [at Camp Nou].
"It's going to be difficult but I know, if we play at our level, we can do something. Let's have high hopes, forget about the game against Wigan and we will see.
"We have nothing to lose. If you go there and you lose, it's no more but, if you get a result, it will be fantastic. We have to just attack them from the start and try to score an early goal. That would worry them so we will see.
"I think anything is possible in football. I saw Barcelona's defeat this week against Valladolid and it just shows they are human. They're a great team but we're not going to have any problem to respect them because we know it's a great team."