No manager in his right mind would choose a derby as a title decider but that is what fate and fixtures have handed Sir Alex Ferguson and Roberto Mancini, after all the false alarms of the last few months when it appeared the result at the Etihad on Monday might prove incidental.

Victory counts for everything now, as Ferguson accepts with surprising candour. "If City win the derby I don't see them losing the title," the United manager says. "Even though they have still got to go to Newcastle, I think they'd have a great chance if they beat us. But that is the only result that's any use to them. Anything else would leave us strong favourites. There's no doubt about that, with two games to go. A home game and an away game. We'd have to be happy if we got a draw but we won't be talking about going there not to lose because that's not the way we play."

This will doubtless be interpreted as mind games in some quarters, as Ferguson attempts to ratchet up the pressure on City not just for the forthcoming showdown but for the remainder of the season, though in reality it is a little late in the day for that. City are under no pressure and Ferguson knows it. They conceded defeat in the title race a few weeks ago, when United opened up an eight-point gap, and the only reason they are now in with a shout is because their neighbours generously left the back door unlocked. It is fair to say Mancini and his players will want to make the most of this unexpected opportunity and have every right to believe in themselves after the 6-1 mauling they handed out at Old Trafford but United are the side who have put themselves under pressure.

No sooner did they assume a lead that even Mancini admitted to be decisive than they surrendered five points in the blink of an eye, with defeat at Wigan and that astonishing eight-goal thriller against Everton. Now they have a derby in which to put things right and, as Everton proved with a listless performance against Liverpool at Wembley a week before the fireworks at Old Trafford, derby games tend to have a life of their own.

Ferguson really did not want it to come to this but it has. "The rest of the season is out of the window now, it doesn't matter," he says. "They are undefeated at home, we have the best away record. It's the ultimate one-off game, with the greatest of prizes for the winner."

All the same, the United manager has had plenty of opportunity to reflect over the last few days how he might have done things slightly differently. Was it really necessary, for instance, to rest Paul Scholes for the Wigan game? "Yes," he says defiantly. "There is no way he can play on a Wednesday after a Sunday game. We know how to handle Scholes, and Ryan Giggs, and you can't overplay them."

He admits to being surprised, to say the least, by Everton's appetite and invention after their barren display at Wembley but does not attempt to deflect attention from his own side's shortcomings. "The chances we had against Everton and the second-half performance we put in, we should have won it comfortably," he says. "With hindsight I should maybe have brought a couple of defenders on anyway just to see it off. I mean, there were only seven minutes to go and we were winning 4-2. Bloody hell, there we go."

It would have been ironic had Ferguson sent on defensive reinforcements, for he had chosen to use the preamble to the Everton match to praise the Rio Ferdinand-Jonny Evans partnership as one of the most solid in the country. Four home goals and two home points conceded later, it did not look it. Ferdinand and Evans found it hard to cope with Everton's movement and workrate and it goes almost without saying that a defence that struggles against Everton is bound to find City's nimble attacking line a handful as well. Here we come to the real crux, for the past few weeks have not simply been about United gaining the initiative and then losing it.

Across Manchester, Mancini's City have been busy regaining their momentum and verve and, though plenty of people apart from Ferguson will be less than thrilled to acknowledge the fact, the man that has made City tick again is Carlos Tevez. The world's most infuriating non-player over winter has popped up again in the spring as fresh as a meadow plant and now seems perfectly poised to perform his usual trick, as perfected at West Ham and then Manchester United, of joining the action at the business end of the season and being influential enough to affect an outcome positively.

The partnership with Sergio Agüero was never supposed to happen – City imagined they would be off-loading Tevez last summer and brought in a second Argentina striker as a replacement – but with nine goals between them in the past three games there is no doubt United will be facing an effective and in-form attacking threat. Mario Balotelli is available again for selection, but Mancini would be bonkers to select him after the way the Tevez-Agüero partnership has flourished in his absence. Ditto Edin Dzeko, despite his two goals in the 6-1 at Old Trafford. It could be pointed out that City's 12 goals in three consecutive victories have come against relatively lowly opposition in West Bromwich, Norwich and Wolves, but winning 6-1 at Carrow Road as well as Old Trafford is something not many other teams will achieve, and on the night City were putting four goals past mid-table West Brom their rivals were losing to Wigan of the bottom three.

At the beginning of the season and even as recently as the start of this month it was being said that United's experience would see them through. City would falter, indeed did falter, but United had the course and distance know-how, the canny manager and the coolness on the pitch to time their run just right. The prediction here was that United would win the title and City would blow up but the picture seems much less clear now the race is perfectly poised again.

United's experience did not appear to count for much against Wigan and Everton, so why should it give them an advantage in an away derby? Perhaps more significantly City have not blown up. They have come back from their wobble, made their peace with Tevez, survived the silliness of Mario Balotelli and against all expectations are in exactly the position that Ferguson has spent most of the season claiming he wanted to be. Within three points of the leaders going into the derby. A win would put City on top with two games to play. City supporters will not necessarily be as confident as Ferguson that Mancini's players can see it out from there but the first part of the deal, a City win in the derby, seems perfectly possible now Tevez has been rehabilitated.

Football writers with a vote have been arguing among themselves in recent weeks about whether Paul Scholes is a legitimate candidate for Footballer of the Year, after appearing for less than half the season. This one thought not and went for Robin van Persie. Yet just as Scholes could still deliver a title to United, so could Tevez for City. Tevez as player of the season was a complete non-starter but Tevez as City saviour in the ultimate Manchester derby, playing for the club he did not want against the club that did not want him, is a story just waiting to be written.