Sergio Aguero scored the only goal of the game as Manchester City beat United in the derby.
Louis van Gaal — already fielding Antonio Valencia as a makeshift right-back — saw Chris Smalling sent off after the centre-back picked up two yellow cards in the first half, and Marcos Rojo later had to be stretchered off.
City duly punished United’s makeshift defence when Aguero scored his 12th goal of the season to hand City a 1-0 victory over their bitter rivals.
England defender Smalling had earned his first booking on the half-hour for blocking Joe Hart’s attempted clearance from point-blank range, and less than 10 minutes later he was shown yellow again for a reckless foul on James Milner.
The victory means City have now won four Manchester derbies on the bounce — something that had not been achieved for 44 years.
It had been suggested before the match that City’s recent poor form meant they no longer enjoyed the supremacy over United that they had enjoyed since Sheikh Mansour started pouring cash into the club in 2008, but this result, which puts City seven points ahead of their cross-city rivals, suggests otherwise.
City failed to show the same killer instinct that Chelsea have shown for most of the year, but they probably should have had at least one penalty.
Michael Oliver waved away shouts for fouls on Yaya Toure, Aguero and a handball by Daley Blind, among others.
The result means Manuel Pellegrini’s three-match winless streak has now come to an end and his side are within two victories of the Premier League leaders with over a quarter of the season gone.
Van Gaal never looked like winning his first Manchester derby once Smalling was sent off and his team lacked a cutting edge up front despite the return of Wayne Rooney.
The United boss’ lack of options at the back was clear throughout. He ended the match with Michael Carrick and Paddy McNair at centre-back after Rojo was carried off on a stretcher with what looked like a serious shoulder injury.
Van Gaal did his best to fan the flames ahead of the match by insisting United remained the biggest club in Manchester despite City’s recent success, and the bitter rivalry between the two clubs was evident from the heated atmosphere inside the stadium.
Van Gaal wanted his team to quieten the home fans with an early strike, but they barely got a sniff of goal in the first 20 minutes.
Despite the prematch hype and bubbling atmosphere, the first quarter turned out to be something of an anti-climax. City were made to chase United, who had the majority of possession but did not do anything with it.
Pellegrini barked orders at the edge of his area, clearly concerned with his team’s performance. The hosts responded with a threatening blitz on the United goal.
Aguero accelerated towards goal, only to be chopped down by Rojo. The City striker got the better of his international teammate moments later but David de Gea saved well.
De Gea then denied Jesus Navas from close range and moments later he made a brave save to deny Aguero.
United were under the cosh. Blind desperately hacked down Toure to prevent the midfielder from unleashing a shot on goal.
The Dutchman went into the referee’s book and Smalling soon followed him for stopping Hart booting the ball from his hands — an act seen more on school playgrounds than in top-flight matches.
Hart somehow avoided sanction even though he appeared to nudge his head towards Oliver’s.
Then came a moment of pure madness from Smalling. Eight minutes after his first booking, the England defender wiped out Milner with a sliding tackle and he received a second yellow.
Smalling trudged off the pitch, and Van Gaal could barely look at the former Fulham man after the error. The Dutchman brought Carrick on to play at centre-half and Adnan Januzaj made way.
City sensed blood and went in for the kill. They were unlucky not to go in at the break ahead. Oliver waved play on even though Marouane Fellaini knocked Aguero over and the referee waved away City’s protests in injury time when Rojo kicked Toure in the box.
After the break, it was much the same story as City had three more penalty shouts rejected. The hosts launched wave after wave of attack and United were struggling to cope.
United’s troubles then deepened when Rojo was carried off on a stretcher after a rash sliding tackle.
With Carrick and teenage defender McNair now operating at centre-half for United, it seemed inevitable City would score. That moment arrived just after the hour. Di Maria failed to cut out a pass from Toure to Gael Clichy and he squared for Aguero, who tapped in from close range.
United briefly came back in to the game. Robin van Persie tested Joe Hart from close range and the England goalkeeper had to be alert to deny Fellaini, who had slipped into the box after a superb ball from Angel Di Maria.
But City, thanks to their numerical superiority, remained very much in charge. De Gea pulled off another top save to deny Stevan Jovetic and Navas struck the woodwork.
All of a sudden, the nerves started to kick in for City — Di Maria blasted a low shot just wide before Fellaini somehow headed wide from eight yards when he was unmarked — but the hosts soon seized control to see out the win.