The decision to call Paul Scholes out of retirement, 225 days after his last appearance in Manchester United's colours, was just the start of an afternoon that veered from the surreal to the fantastical. Three goals up on their local rivals' turf before half-time, and enjoying a numerical advantage after the early dismissal of the opposing captain, United appeared to be on the way to exacting statistical revenge for the famous 6-1 defeat at Manchester City's hands on their own home ground in October.
But nothing about this third-round tie was destined to be predictable, starting with the moment, an hour before kick-off, when United announced that Scholes had been re-registered as a player and would be taking a place among their substitutes for this high-octane local derby.
"When I saw him on the bench it was incredible," Roberto Mancini said. But it was not even the most incredible aspect of this tumultuous contest, in which United held on by their fingertips to win 3-2 against the assault of a 10-man City side cleverly reorganised by Mancini after the interval to compensate for the absence of Vincent Kompany, dismissed for a two-footed tackle on Nani with only 12 minutes gone.
Mancini declared himself disappointed with the result but happy with the second-half performance of his depleted team, who brought fighting spirit to a shrewd tactical switch which enabled them to terrify their opponents. Sir Alex Ferguson criticised his players' carelessness after the interval, the worst example coming from Scholes, who replaced Nani, played a sloppy pass from a throw-in and saw James Milner steal the ball before sending in a cross from which Sergio Agüero narrowed the deficit to a single goal.
The decision to recall the 37-year-old Scholes, seven months after his last appearance against Barcelona in the European Cup final at Wembley, seemed to indicate a desperate shortage of midfield resources at Old Trafford. What would Paul Pogba and Ravel Morrison, the stars of last spring's FA Youth Cup winning side, be thinking of the decision to give a 677th appearance in the club's colours to a man who had a testimonial match in August before settling into the life of an assistant coach without portfolio at United's training ground? Since the No18 he wore for the best part of two decades was reallocated last summer to Ashley Young, Scholes was given the No22 shirt, the one he wore when breaking into the side.
Ferguson said: "He came to see me and said: 'I regret retiring'. He said he wanted to come back, that he was missing it too much. There are no negatives for me. The players are delighted, I'm delighted, the fans are delighted. For the last few weeks he has been stepping up his training. He has been taking part in our training sessions during the week. Paul has been training very hard with the reserves and doing a lot of work in the gym. It is a terrific addition to our squad at a very important part of our season. He's a fantastic footballer and there's been no real loss to his fitness, which is important."
"I've been pretty clear since I stopped playing that I miss it," Scholes said. "I'm delighted the manager feels I can still make a contribution to the team and I'm looking forward to playing my part in trying to bring more success to this great club."
City had begun the match promisingly, with eight minutes of unbroken attacking. In the ninth minute United ventured forward for the first time, Wayne Rooney feeding Phil Jones, whose pass to Antonio Valencia ran into touch. A minute later Rooney fed Valencia himself and ran on to meet the cross with a header past Costel Pantilimon, City's reserve goalkeeper. Would Joe Hart, sitting out the match on the bench, have reached it? Mancini could have stationed Bert Trautmann on one side of Pantilimon and Joe Corrigan on the other, and Rooney's tremendous effort would still have silenced the home supporters.
Kompany's red card followed two minutes later, flourished by Chris Foy after the City defender had dispossessed an understandably hesitant Nani with an intervention that was judged by the referee to constitute dangerous play. For most old-timers watching in the ground or on television the instinctive reaction would have been a simple one: "Great tackle!" But the game's ruling bodies now seem determined to stamp out reckless challenges, which means taking action against some examples that would once have been applauded as merely forthright.
For the remainder of the half City were a demoralised mess, conceding goals first to Danny Welbeck's hooked volley and then to Rooney's header after Pantilimon parried his penalty kick. But at the interval Mancini replaced two attackers, David Silva and Adam Johnson, with two defenders, Pablo Zabaleta and Stefan Savic, reorganised the side into a 3-4-2 formation, encouraged his players to go forward and was rewarded with two goals and a demonstration of relentlessly positive play that put the visitors firmly on the back foot and threatened to salvage a replay.
City's own manpower problem, caused by the departure of Yaya and Kolo Touré for the Africa Cup of Nations, provoked another unexpected arrival. With eight minutes left, the ineffective Samir Nasri was replaced by Owen Hargreaves, who endured four injury-wrecked season with United before moving across Manchester in August. The Canadian-born former England international took up a position in central midfield but had no opportunity to make an impact.
By that time Scholes had found the rhythm of the game and as the final whistle went he was meticulously puncturing the optimism of City's last frantic assault by taking possession on the left of the home penalty area and measuring a careful, pressure-relieving pass up the line to Ryan Giggs.
Scholes to Giggs: combined ages, 75 years. And since United announced that their new No22 will remain on the playing staff until May, you may well be reading those words again as this acid trip of a season continues on its extraordinary course.