United boss Ferguson (right) got the better of City manager Mancini last season
BBC Sport football expert
When Manchester City and Manchester United meet, there is always plenty riding on the outcome - but it is hard to overstate the significance of the game at Eastlands on Wednesday.
There will be sub-plots on and off the pitch, while both bosses, Roberto Mancini and Sir Alex Ferguson, will see this game as a perfect opportunity to strike an important blow in the battle with their neighbours.
If Manchester City win, they will be level on points with United, but if the result is reversed it will obviously open up a six-point gap that Mancini and his team might just struggle to bridge, with all the subsequent pressure that will bring for the Italian.
The last thing this magnificent manager [Ferguson], who has just celebrated 24 years at Old Trafford, would want is to take his leave of the club with City on top in the Manchester rivalry
City have put themselves back on an even keel after losing at home to Arsenal, when I actually thought they played well with 10 men, and the defeat at Wolves, when every man and his dog was castigating them for the perceived lack of team spirit.
After the Europa League reverse against Lech Poznan last Thursday, City and their manager needed a result desperately on Sunday at West Brom - no easy task this season - but they did it well and the mood in the camp will be lifted now.
Mancini said on Tuesday that a place in next season's Champions League may be the realistic goal but it remains to be seen if this can be achieved. One thing is certain, if City's Abu Dhabi backers keep putting money in at the rate they seem happy to do, it will bear fruit eventually.
And the opportunity to make even further progress is there. The teams who dominated those top-four places for so long - Chelsea, Manchester United, Arsenal and Liverpool - are not as good as they were. Liverpool, of course, have slid even further back into the Premier League pack.
There was talk only a few weeks ago, especially after City beat Chelsea, that the balance of power in Manchester was shifting. It certainly has financially, with the untold riches at Eastlands, but United are still ahead of their neighbours when it comes to the football, something Ferguson will be desperate for them to emphasise on Wednesday.
My view is that City, if the investment continues at current levels, are more or less an unstoppable force once the manager in charge gets the formula and balance right. The job for United, and of course for the other clubs who want silverware, is to stall this progress for as long as possible.
Tevez left United to join City to much fanfare on the blue side of the city
Ferguson will want to make sure City never overtake United on his watch - and I suspect he reminds his players of this on a regular basis.
The last thing this magnificent manager, who has just celebrated 24 years at Old Trafford, would want is to take his leave of the club with City on top in the Manchester rivalry. Every derby is a chance to underscore United's superiority.
So to the game itself - and in some respects Mancini and his players may just be feeling this is the perfect time to play United. They have struggled for form, got a late winner against Stoke and an even later one against Wolves, have their best player in Wayne Rooney out injured, while there has been illness and injuries in the camp.
But, and this is a big "but" as far as City's chances are concerned, United are masters of pulling out results when they need them. Remember how City lost three games to their arch-rivals in injury time last season?
After throwing away two points at home to West Brom in October, United have turned the corner in terms of results, especially when you add in the recent 2-0 win at Old Trafford against Tottenham, but they have not turned the corner in terms of form.
This is their biggest test in the Premier League this season but, for anyone tempted to make United underdogs, they have more often than not got the result they want when it comes to the crunch.
United have not hit top form this season but study the results. They have not lost a game yet and remain, along with Chelsea, one of only two teams I think can win the Premier League this season.
City will look to Carlos Tevez to provide their inspiration, as he has done so many times since his move across Manchester from Old Trafford. He was behind Rooney and Cristiano Ronaldo in the pecking order at United, perfectly understandably, but he is the main man at Eastlands and has thrived on it.
Tevez is a fabulous player and I think the other players look up to him. Sometimes you have players in your dressing room that you look across to and think: "I'm really glad he's playing for us today."
When I was at Liverpool, we had plenty but it was Graeme Souness and Kenny Dalglish who the others looked to, not only because of their ability on the pitch but the desire to win, the strength and the determination to give you something when your back was up against the wall.
United had Rooney and Ronaldo to do it when they played together at United. Tevez is now that figure for City and so much will rest on him on Wednesday.
He has grown in stature at Eastlands. The other players look up to him, the supporters love for him his ability and his decision to leave United and join City, and he is the one they will hope will give them something extra.
The bookies might say this game has got draw written all over it - I might even agree with them - but what an opportunity for Manchester City's players to show they are backing their manager and to make a big statement about their season by drawing level on points with United.
And for United, this is an opportunity to stall the Eastlands march and once again assert their footballing authority in Manchester.
Alan Hansen was talking to BBC Sport's Phil McNulty
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