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* Manchester United deserved their victory in a cracking, incident-packed match between two teams who are far and away the best in the Premier League at this point in time.
United have never lost a match in which they have taken a two-goal lead under Sir Alex Ferguson and, for arguably the first time this season, they played with a visible game plan at City as they stole all three points at the death.
It was a previously typical United performance - hinging on ruthlessly effective counter-attacking and stretching the game on the wings - and after Ashley Young's goal was wrongly disallowed for offside, Ferguson will feel justice has been done as his team stride six points clear at the top.
* If United had been held to a 2-2 draw, we would never have heard the end of how Young's 'goal' would have given the Reds an insurmountable 3-0 lead. So perhaps the neutrals among us should be relieved that Robin van Persie settled the match.
The replays of Young's strike provided conclusive evidence that the winger wasn't offside when Van Persie struck his shot, and just 60 seconds after United should have wrapped up the points, City grabbed a lifeline through Yaya Toure.
Ferguson reflected after the game that it would have been a "killer" moment if United hadn't gone on to win and, given how close last season's title race was, it could have proved to be enormously crucial.
* On a day in which several of City's star players failed to fire, Joe Hart must ultimately take the largest portion of the blame for a defeat that saw the champions lose their unbeaten record this season and also suffer their first Premier League home defeat for the first time in 38 matches.
While Hart should possibly have made more of an effort to reach Wayne Rooney's scuffed shot for United's opening goal, there can be no doubts about the goalkeeper's failure to command his area in the build-up to Robin van Persie's late free-kick.
As a loose ball rolled through into the City box, Gael Clichy was begging his 'keeper to react, but Hart remained rooted to the touchline, failing to anticipate the danger. Of course, Clichy also played his part by conceding possession to Danny Welbeck - and Samir Nasri's can-can couldn't as the midfielder deflected Van Persie's drive into the net - but Hart could have prevented all this by commanding his area in the initial phase.
Hart also shifted his weight onto his wrong side just before Van Persie struck the free-kick and, on a day in which United scored with all three of their shots on target, the England number one can hardly have been pleased with his performance.
* Roberto Mancini must also accept responsibility for City's defeat after again choosing not to partner Sergio Aguero and Carlos Tevez in attack.
While Mancini obviously needs to rotate his squad to ensure every player stays fresh and injury-free throughout the season, Sunday was the right occasion to pick his best team. City have won six of seven matches in which Tevez and Aguero have started together this season and through their movement and understanding, the two strikers offer a far greater threat than Mario Balotelli or Edin Dzeko.
Balotelli wasn't entirely awful in his 52 minutes on the pitch, but his movement was poor and ponderous and, after one particularly wasteful back heel, Mancini decided to haul the striker off.
Within ten minutes, Tevez had provided an assist for Yaya Toure and demonstrated the energy and purpose that could have been vital if he had been on the pitch from the start. It wasn't a tricky decision for Mancini to make - his best first XI has been obvious for some time now - yet he ignored the obvious selection to indulge a player who has scored only one Premier League goal all season.
* After United's dreadful display in their costly 1-0 defeat at the Etihad Stadium last season, all eyes were on Sir Alex Ferguson's team selection before kick-off, with the manager no longer able to order Park Ji-Sung to carry out a man-marking job on Yaya Toure.
That tactical instruction highlighted United's fear and lack of intent in April, but on this occasion Ferguson's starting line-up showed a lot more purpose. The return of Antonio Valencia was a huge boost and the selection of Tom Cleverley - who was rushed back from a thigh strain - underlined his role in the first XI.
There was an excellent balance to United and the pace of Valencia and Ashley Young enabled them to counter quickly and to great effect. In April, Ferguson wanted to avoid defeat; on Sunday he set up his team to secure victory.
* Although United were impressive following Rooney's first strike, City were actually the better team in the opening stages. In the first 12 minutes the champions recorded 76% possession and their patient build-up play almost resulted in a goal when David Silva's sublime pass picked out Clichy, who crossed for Balotelli to blaze over.
But the following ten minutes saw United gain the upper hand and Rooney's dragged effort knocked the stuffing out of City and reinforced the visitors' belief.
* Rooney's finish certainly wasn't worthy of any goal of the season award, but the speed at which United broke was breathtaking.
It took only ten touches and 15 seconds for United to move the ball from their own box into the back of the City net, with Young and Van Persie linking up superbly on the left to leave Pablo Zabaleta in no man's land.
* Rooney's first owed more to the brilliant United move than any specific mistake by City, but the striker's second could in part be blamed on David Silva. Gareth Barry indicated for the playmaker to track Rooney as he moved across to help out Clichy, but Silva ignored the instruction and the United forward was left unmarked to slide home.
Could Kolo Toure have got closer to Rooney? The defender didn't have anyone else to mark in the box and perhaps should have anticipated the striker's run, but ultimately it was Silva who failed to do his job.
* City had conceded just two goals in the six Premier League matches before Sunday in which Vincent Kompany and Matija Nastasic had been the starting centre-back pairing, and the captain's forced substitution after just 21 minutes rattled the champions.
Anyone who saw Toure's lack of excitement in last season's title celebrations will have realised that the defender is more car salesman than footballer in Manchester these days, and it seemed odd that Mancini chose not to send on Joleon Lescott, despite the left-footed imbalance that would have caused.
However, it should be said that Toure settled after a nervy half-hour and his presence was largely unnoticed in the second half as City fought their way back into the match.
* David De Gea's presence was certainly noticed, however, as he was praised for a superb double save before Yaya Toure eventually stroked the ball home for City's first on the hour mark. But could De Gea have done better with Tevez's initial strike?
The striker's shot was at a good height and in a good position for De Gea to push the ball around the far post, but instead the Spaniard parried it back into the middle of the goal for Aguero to shoot.
Although De Gea did well with several crosses, he didn't have the complete error-free performance he would have hoped for in Anders Lindegaard's absence.
* Ferguson conceded that City were the better side as the second half developed and this was mainly brought about through their improvement in the wide areas. In the first half, the champions were far too narrow as United's centre-backs repeatedly picked out the wingers with long balls to set the team on the counter attack.
But after the interval, Clichy and Zabaleta gambled by pushing further forward, forcing Patrice Evra and Rafael to sit back and defend. Nasri also drifted out to the right and this helped City to get the ball into the box more often.
* Van Persie's last-gasp winning goal highlights City's failure to add a match-winner to their squad in the summer. The striker was long linked with a move to the Etihad, but eventually rejected the champions' offer in favour of a move to United.
There was obvious tension between Mancini and City's 'football administrator' Brian Marwood during the transfer window, with the manager frustrated at the club's failure to attract his top two targets, Van Persie and Eden Hazard. And moves for Javi Garcia, Scott Sinclair and Jack Rodwell have not added the quality City required to increase last season's narrow gap at the top and also improve in the Champions League.
Daniele De Rossi is still thought to be a target for January, and City perhaps need a stellar signing to inject renewed belief into their big-spending project.
* One of the club's current big names, Yaya Toure, again looked worryingly subdued on Sunday as he failed to drive at the United defence with the devastating effect he demonstrated in April. Toure seems to be lacking confidence lately and also looks to be carrying a little extra timber, which is perhaps the reason for his loss of pace. To compound a poor afternoon, he was forced to go off after injuring himself in a challenge with Tom Cleverley. That's right, Yaya Toure injured himself fouling Tom Cleverley.
* While City's top players failed to perform, Rooney had a brilliant game for United in the number 10 role he now favours. With United attacking at speed down the flanks, the team was set up perfectly for Rooney to act as a pivot behind Van Persie. Ferguson said recently that he wants more goals from Rooney and the 4-2-3-1 system the manager played on Sunday is perfect for the forward to arrive late in the box to score, as he did for his second strike.
The position behind Van Persie also allows Rooney to drop into midfield when United are under pressure, which he did to good effect in the second half.
* So what does this mean in the title race? United have struggled all season for consistency (in terms of performances, at least), but looked remarkably comfortable for long spells against City. Ferguson's selection and formation showed that he knows his strongest line-up and how he wants the team to play, and United repaid the manager with a convincing display.
Conversely, Mancini was lacking in his team selection, forgetting that Tevez and Aguero are by far his best attacking partnership. It was embarrassing for both Mancini and Balotelli when the manager accepted his mistake shortly after half time, and he seems to be tinkering for tinkering's sake of late. But with City now out of Europe, Mancini may settle on a more consistent first XI.
City certainly have an advantage over United in that respect but, as I've already said, they failed to significantly strengthen their squad in the summer and one of the team's key players, Yaya Toure, appears worryingly subdued, while Silva has struggled with injuries this campaign.
We're still a long way from anything being decided - and United threw away an eight-point lead in the final six matches last season - but one thing's for certain, it's going to be another exciting ride.
* Rio Ferdinand could easily have been blinded by the coin that was thrown at his face. Is it even possible to educate against such idiocy?
Matt Stanger - he's on the Twitter.