The Frenchman would be advised not to show his face around the club for a few days after failing to show his face when it mattered most in this epic derby. Such key moments can unlock trophy cabinets.
Trailing 2-0 at the break, the champions had rallied impressively. Halfway through the four minutes of added time, City just had to repel a Robin van Persie free-kick to protect their 37-game unbeaten league home record, to ensure United were only three points clear.
City's wall was initially a gang of four. Gareth Barry and Edin Dzeko, the taller players, protected Joe Hart's near post. Then came Nasri and Carlos Tevez, who suddenly broke away from the wall, seemingly to follow Wayne Rooney. And then there were three.
With Tom Cruise looking on, Nasri decided to audition for Eyes Wide Shut.
When Van Persie drove in his free-kick, Nasri slipped behind Dzeko, dangling out his left foot as his sole contribution to the resistance movement. In terms of keeping the wall intact, it was rather like replacing a foundation stone with an eclair. And then there were two and a half.
Van Persie ran behind him, past the City fans screaming "f*** off" as he passed en route to the away section.
All the United players, barring David de Gea, piled on him. One of the last to arrive was Rio Ferdinand, who was hit by a two-pence piece that pierced the skin above his left eye. As the blood trickled, a fan in City colours invaded the pitch, but his passage towards Ferdinand was blocked by the quick-thinking Hart.
As City seethed, United celebrated. Alex Ferguson was the last of the United staff to leap from his seat, but his smile and arms raised encapsulated his joy and appreciation of the goal's significance. City possess the strength to fight back, reeling United in as they did last season, but it felt a huge moment.
It was also a reminder, if any were needed, of exactly why Ferguson pursued the goal-taking, responsibility-taking Van Persie in the summer. The Dutchman was walking back to the halfway line, a grin on his lips, another handsome repayment made on his £24m fee. To think Nasri cost City roughly the same sum from Arsenal.
Mancini was understandably aggrieved by Nasri's lack of robustness at such a critical moment, at a lapse of concentration and character, at all the second-half recovery work thrown away.
Yet Mancini was also culpable, having decided to start with Mario Balotelli. Hailed by his agent as being as valuable as the Mona Lisa, Balotelli was more a still life at times. He was not alone. City were outpaced and outwitted by United in the first period. The visitors were sharper, hungrier, more clinical.
Ferguson's 4-4-1-1 system gave them numbers in the middle, including Wayne Rooney selflessly dropping back, and pace down the flanks. If Rooney was the brains and heartbeat of United's performance, Michael Carrick was not far behind, refusing to be bossed by the likes of Yaya Toure.
Too cautious on their last visit here, Carrick set United's assertive tone early by catching Sergio Aguero. The opening minutes resembled a lighter being lobbed into a box of fireworks. Rooney clattered David Silva, then Toure. Ferdinand upended Aguero. City dominated the early exchanges.
Balotelli sent in a free-kick which was pushed behind by De Gea and then wasted Gael Clichy's excellent cutback.
United simply soaked up the pressure, hitting on the counter after 15 minutes. Ashley Young, collecting possession in his own half, headed the ball over Zabaleta and ran on to the chested return from Van Persie, a wonderful interchange.
Young accelerated deep into territory vacated by Zabaleta, before transferring the ball inside to Rooney, who scored with a low, scuffed shot.
The half continued to darken for City. Vincent Kompany hobbled off, rather mournfully handing the armband to Zabaleta. Joleon Lescott could have been forgiven for worrying further about his City future when Kolo Toure was sent on to partner Matija Nastasic, although Mancini may have been wanting to avoid pairing two left-footed centre-halves.
Kolo Toure was soon sliding in vain as Rooney struck again after 28 minutes. Antonio Valencia sent Rafael scampering upfield and the Brazilian's cross skimmed through Clichy's legs to Rooney, who placed his shot across Hart and in. For a talented, industrious player precipitously written off far too frequently, Rooney's second took him to 150 goals in the Premier League, a mark eclipsed by only Thierry Henry, Alan Shearer, Robbie Fowler, Andy Cole and Frank Lampard.
As Rooney's stock rose, Balotelli's continued to slide. Five minutes after the break, Balotelli was withdrawn, the Italian heading straight down the tunnel.
Tevez was immediately influential, running the channels, running everywhere, seeking to apply the jump-leads to City's stalled engine. Yet United should have been over the horizon when Young turned in the loose ball after Van Persie's shot hit the post. Young was harshly ruled offside.
A scoreline that should have been 3-0 was soon 2-1. De Gea saved superbly from Silva and Tevez at close range, but Tevez stayed composed, turning and picking out Yaya Toure, who scored with a low shot. Game on. City fans lit a blue flare, the smoke floating across the pitch like a malevolent cloud.
With four minutes remaining, and City supporters beseeching their players to find an equaliser, United failed to clear a Tevez corner and Zabaleta levelled with a low strike. But Tevez then gifted Van Persie his free-kick chance with a foul on Rafael. Having been booked for dissent, Tevez could easily have gone for kicking out at Phil Jones.
As Ferguson walked off, a City fan waved a placard bearing a picture of a laughing blue moon with the words: "Hey, Fergie, you ain't heard nothing yet." But the noisy neighbours were falling silent and United fans were chorusing: "Always look on the bright side of life." Nasri wasn't looking. (© Daily Telegraph, London)