With a 5-0 FA Cup mauling at Nottingham Forest coming on the back of a run of one win in 13 Barclays Premier League games, the final four meeting with City is a welcome distraction for Allardyce's side.
The dismal December run means West Ham sit 19th in the league but Allardyce will not be too downbeat if his side fail to pick up a rare win at the Etihad Stadium and just wants to remain in the tie.
"We don't have to beat them on Wednesday night," he said.
"We have to come back with something realistic to beat them at home if we can. We have to stay as focused as we can and we have to nullify what is a great attacking force at home. Everybody knows what that is and how good that is."
Allardyce has cited the example of Crystal Palace, who pushed City all the way in a 1-0 defeat in December.
He said: "We have to work along the lines of what Crystal Palace did recently which was be very well organised, be a very well structured team that makes it extremely difficult for the talents that Manchester City have got.
"Hopefully we can do that on Wednesday night and when we come to the end of the game it's only half-time at that stage."
The 59-year-old fielded an inexperienced side at Forest with just 14 available first-team players and, with his current injury plight, Allardyce cannot enjoy the semi-final experience as much as he would like.
"I would have preferred it to come with a lot more players fit and not in this position that we're in at the moment but we have to face the reality of what comes along in football," he added.
"We are in a very exciting cup tie with a massive, massive feeling of joy at the end of it if, as big as the game is and the underdogs that we are, we believe that we can get there and believe that we can get to Wembley.
"With a bit of luck, a bit of fortune and the right amount of organisation, desire and commitment from the players we feel as though we can give it our best shot.
"We don't have the same pressure we experience in the Premier League that is for certain. It is a competition where we know at this time it is over two legs, in the earlier stages you say 'it is either win or we are out'."
West Ham's co-chairmen David Sullivan and David Gold issued an open letter to fans on Monday confirming their support for Allardyce, who was happy to receive the backing.
"I'm obviously pleased by the united front we are trying to keep under these difficult times at the moment," he said.
"When it comes public from the co-chairmen it is nice to see and hear. From our point of view we are all understanding the difficult position we are in at the moment.
"We all have to work through it together in the hope that we turn it around into a positive reaction in terms of our results."
Roger Johnson, signed on loan from Wolves for the remainder of the season, will go straight into the squad which travels to Manchester as the 30-year-old adds much-needed strength to Allardyce's defence.
"The one big thing about Roger is his Premier League experience at Birmingham City," he said.
"His height, which has been a weak point for us recently, and a desire to come and show he has still got the capabilities of coming to play at this level.
"He is not cup-tied, so going into one of the biggest games of the season for us with someone with his experience against one of the biggest sides in the Premier League is a help.
"If you look across the board at Man City, aerially we have to be very competent and Roger will be a great help in that area particularly.
"He is very determined and excited about the opportunity he has got after being basically frozen out at Wolves."
Allardyce confirmed club-record signing Andy Carroll is closing in on a long-awaited return to action but will not be ready for Manchester City.
Skipper Kevin Nolan is suspended with James Tomkins, Winston Reid, James Collins and Ricardo Vaz Te also absent.
Allardyce is hopeful Mark Noble may have recovered from a calf injury to play some part in the tie.
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