The cultivated air and charming grace, the notion of an ageless refinement, are a distraction.

Roberto Mancini, reduced to his most essential quality, is a hard, ruthless manipulator. He is only the latest manager to challenge Sir Alex Ferguson's vehement dominance of the English game, and no others have landed anything more than fleeting blows, but Mancini has already earned the wariness of the Manchester United manager.

The two men are involved in a tense, polite conflict, when their inclinations would normally be to resort to brawler's instincts. Ferguson tells of his admiration for the other man's authoritative nature, while the Italian pays constant respect to his contemporary, and made a point of wishing him a happy 70th birthday at the turn of the year. Yet Mancini's playing career was littered with angry confrontations with team-mates – he once stripped to the waist as he waited for one to return to the dressing room so that they could continue their skirmish – but he was also cunning enough to strike the right relationships with owners, chairmen and managers to consolidate his own power.

This combination of aggression and scheming has been central to the progress City have made since the Italian replaced Mark Hughes at the Etihad Stadium three years ago. Mancini encourages his players to be full-blooded in training, and several fights have been caught on camera, but it is the adamant way that he dealt with Carlos Tevez, the Argentinean striker ostracised after a fall-out, that reflects the extent of the manager's control.

Mancini is a complex individual, capable of indulging Mario Balotelli while bluntly spurning other players; of the self-regard of hanging two portraits of himself in his office; of a superstitious mind that saw him rush across the room to dip his fingers in some wine that spilled from a glass, then dab the liquid behind his ears – to ward off bad luck – in front of Kenny Dalglish and Steve Clarke, following City's win over Liverpool on Tuesday.

This calculating nature was at work when Mancini declared that his team's title challenge might be constrained by the board's refusal to sanction more spending, and that he might be unable to name a full substitutes bench in tomorrow's FA Cup third round tie against United. Sheikh Mansour has spent £500m on players since buying City in 2008, which belittles to an absurdity the notion that Mancini has been hampered in his work.

The lavish spending must come to an end if the club are to comply with UEFA's new financial fair play regulations, but a manager who can select a forward line from Sergio Aguero, Samir Nasri, David Silva, Edin Dzeko, Adam Johnson and Balotelli is hardly in a predicament. Yet Mancini's resourcefulness will be tested in the coming weeks, with the Toure brothers playing in the Africa Cup of Nations and Balotelli, Dzeko, and Nasri unwell.

Mancini fell out with Garry Cook, City's former chief executive, when the two men disagreed over signing targets. The manager now wants to bring in a new striker and midfielder – most likely Daniele De Rossi, of Roma – but the likes of Tevez, Wayne Bridge and Nedum Onuoha will have to be sold first. Having shrewdly developed a strong relationship with Sheikh Mansour and Khaldoon Al Mubarak, the chairman, and established City at the front of the championship race, Mancini is unlikely to be rebuffed.

It is the balance of power with United that is more precarious, with their rivals bringing a sense of humiliation with them to the Etihad Stadium. The 6-1 scoreline that City registered at Old Trafford earlier in the season will still pain Ferguson, but consecutive defeats by Blackburn Rovers and Newcastle in the Barclays Premier League have cast United as the team in troubling decline.

Ferguson – who late last night was busy quelling suggestions that he was keen to sell Wayne Rooney – suspected yesterday that City might have been planning a ruse in announcing that Yaya Toure would not be playing, but he and his brother will have left to join up with the Ivory Coast squad by the time tomorrow's game kicks-off. The absence of Yaya Toure, a figure of power and guile, will leave City's side bereft.

Mancini seemed to dismiss the prospects of Owen Hargreaves, the former United midfielder, being an adequate replacement with the cruelly arch remark that "he is not Yaya". Nigel de Jong, the Dutch internationalist, can provide some brawn and remorseless aggression, but he does not combine it with Toure's aplomb going forward.

Neither team will be at full strength, and the third round of the cup seems a minor setting for a pivotal encounter, but there is more at stake than progress towards Wembley. City won the FA Cup semi-final between the two sides last season; a result, and performance, that confirmed their place as United's pre-eminent challengers. The 6-1 defeat seemed to establish their dominance.

A win for United would arrest the sense of them being a diminishing force, while a victory for City, particularly a depleted side, would seem like a decisive strike. That prospect is one for Mancini, a streetfighter at heart, to relish.