City were dumped out of the quarter-finals after losing 2-1 to Athletic at the Etihad, upset by the same team who shocked them in last season's final.
Nasri is hoping that the anger and frustration the players feel will translate into a shock victory of their own when they face Barcelona on Wednesday night.
City trail 2-0 going into the second leg of their Champions League last 16 tie, and they face a daunting task at the Nou Camp to turn that scoreline around.
Asked how he felt after 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."
A Jordi Gomez penalty and James Perch tap-in put the Latics 2-0 up, but City rallied, with Nasri pulling one back and the hosts laid siege to the Championship team in the last 30 minutes but failed to find the equaliser.
Asked what the approach against Barca should be, Nasri 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's last trip to Barcelona was with Arsenal four years ago, and they were hammered 4-1 with Lionel Messi scoring all four.
Nasri 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...."