Roberto Mancini's City have conceded 22 goals in all competitions so far this season, having let in only 29 league goals in the entire title-winning campaign last year. Allardyce has seen his side pick up 14 points from their opening nine league games, but despite pinpointing City's problems at the back, he is in no doubt that his side need to perform well to get a result.
He said: "I'm surprised (by City's start) but yet again there have been a few changes and a few players he has lost through injury and that disrupts what was his renowned back four for last season."
He added: "He ( Mancini) has tried a different style, a different system, he has lost (Nigel) de Jong who used to give them that protection.
"It is a bit more encouraging - I watched Swansea last week and they were very impressive against them but still at the end of the day they didn't get anything out of the game.
"When you are driving down the motorway or leaving Upton Park tomorrow it is no good saying 'what a good performance' if you have lost, so you have to get a result if you have given a good performance and that is what we are looking for."
Allardyce knows his players will have to make the most of the limited possession they are likely to enjoy against Mancini's side, as well as keeping an eye on all of City's attacking talent.
"I think that you set your stall out tactically against these teams to stop the major threats they have," he said. "They have a lot of talented players and that is one of the key reasons you have to focus on trying to nullify that talent if you can and then maximise your possession.
"If you're going to be limited in possession against the big boys you have got to make sure you have got the quality to maximise that and try and expose one or two of their weaknesses."
A partisan home support could also work to the Hammers' advantage, according to Allardyce, who added: "The crowd will be up for it, we will fill the place and they will be excited. They will anticipate we will give a good performance and that is all you can ask for from your players against a Manchester City side, against the champions."