The two Manchester sides meet at the Etihad Stadium on Monday evening and United will be well aware that if they suffer defeat, Roberto Mancini’s side will regain top spot on goal difference.
Nani told The Sun on Sunday: “We’ve been careless in recent games and only have ourselves to blame for being in this position.
“We should have been going to City with an unassailable lead. But, instead, we are facing a very difficult game with everything at stake.
“Shame on us. There is no excuse. A year without a title for a club like Manchester United would be a very bad year for everyone at the club. It would be a disaster.”
|14/1||Nani is 14/1 with Paddy Power to score the first goal in Monday night's Manchester derby.|
The Portuguese winger joined from Sporting Lisbon in 2007 and has admitted that he wants to spend the rest of his career at Old Trafford.
“I left Portugal for a great personal challenge and to improve as a player," added the 25-year-old.
“There have been times when it’s not been easy but they treat me well here and, step by step, I am fulfilling my dreams. Manchester United is my home.”
Nani has attracted interest from Italian giants Juventus and AC Milan, but has his heart set on a contract extension that would keep him at the club until 2016.
Nani said: “The manager has immense faith in me and that’s all that matters.
“My agent is negotiating with the club on a new deal and things are going very well. I hope at the end of the season an agreement will be reached.”
The winger’s career has gone up a notch since the departure of Cristiano Ronaldo. The two Portuguese internationals are still in touch, but Nani does not welcome comparisons between them.
He added: “I am always in contact with Cristiano.
“He has been a great help to me since I came to the club, along with the manager and Carlos Queiroz. But I don’t like the comparisons with Cristiano as a player.
“We are different and have attributes to our game that aren’t similar. But, unfortunately, the public frequently compare us.
“What I can tell you, though, is Cristiano is still a huge United fan and is desperate to see us retain the title.”
Nani is hoping to shake off an ankle knock in time for the Manchester derby, and insists United remain masters of their own fate.
He added: “It has been a season of many ups and downs but our destiny still remains in our own hands.
“However, we won’t be going looking for a draw — even though we find ourselves three points better off than City. It’s not our mentality, it doesn’t come into our thinking.
“That [6-1] defeat [by City at Old Trafford in October] was extremely painful and people were quick to write us off afterwards.
“But we have shown immense resilience, proved people wrong and head to City leading the Premier League.
“There is no room for complacency but no time to dwell on what happened earlier this season.”