City crashed to the little Latics in the cup yet again on Sunday as they suffered an unlikely repeat of last year's final loss in the last-eight stage.
They now have little time to refocus following the 2-1 defeat as they prepare for the huge challenge they face at the Nou Camp on Wednesday.
City take on the Spanish champions in the second leg of their last-16 tie needing to overturn a 2-0 deficit to reach the quarter-finals.
Asked how he felt about the Wigan result, Nasri said: "Hungover. It was a bad performance, we didn't play well.
"The big disappointment is that it is twice now that Wigan have beaten us in the FA Cup. It is really difficult to accept.
"It is like - the FA Cup, they gave it us on a plate and we refused to take it.
"I really don't know. I cannot find the words because I am really disappointed. Thank God we play a big game on Wednesday. It is the best way to forget this game and be focused on the next one.
"At training we are going to think Barcelona and try to think positive and get a result there."
City failed to respond to the high-energy game played by Wigan at the Etihad Stadium and fell 2-0 down after a Jordi Gomez penalty and James Perch tap-in.
They did rally in the last half-hour and pulled one back when Nasri's low strike beat the dive of Scott Carson.
They then laid siege to the Wigan goal in the closing stages.
It proved too little too late to avoid a cup shock but Nasri wants to carry that momentum forward to Barcelona.
Asked what the approach should be, the France international said: "Exactly the same way we finished the (Wigan) game.
"If you go there and you lose it is no more but if you get a result it will be fantastic.
"I think we have nothing to lose - just attack them from the start and try to score an early goal.
"For a game like this you cannot not be focused from the start.
"It is an amazing game and we can do something fantastic if we change the result.
"But we have to be focused and take this (Wigan) game as a lesson."
Nasri does not have particularly happy recollections of the Nou Camp having been in the Arsenal side thrashed 4-1 by Barca - or more particularly Lionel Messi - four years ago.
He said: "It is not a good memory.
"But at the time I thought they were better than they are today.
"We played against Messi, who at the time was unplayable. He scored four goals against us.
"I hope it is not the going to be the same."