Cristiano Ronaldo was sent off for the second time in a Manchester derby, but Wayne Rooneys first-half strike was enough to hand United all three points at Eastlands.
Ronaldos two yellow cards the second for a bizarre deliberate handball meant the Reds had to play the last 22 minutes with 10 men. But City didnt threaten until stoppage time and United held on to claim a deserved derby-day win.
Sir Alexs men controlled the game from start to finish, proving class and ability on the pitch counts for far more than wealth and ambition off it. Certainly, when Citys new owners took over in the summer, expectations in the blue half of Manchester rose astronomically. Almost immediately, the owners outlined a wish to win the Champions League within just three years. On the evidence of the 151st Manchester derby, however, City still have a lot of work to do.
Dimitar Berbatov made a surprise return to the Reds starting line-up after missing Uniteds last two matches with a hamstring injury. His inclusion alongside Wayne Rooney meant Carlos Tevez had to be content with a place on the bench.
Elsewhere, Ji-sung Park was given the nod ahead of Ryan Giggs and Nani on the left side of midfield, while 18-year-old Brazilian Rafael started at right back in his first Manchester derby. He showed little sign of nerves, darting down the wing early on to add an extra man to the Reds attacks.
It was from the right that Uniteds first real chance came, although it was Ji-sung Park and not the young Brazilian who fizzed the ball across the Blues box. It reached Rooney at the penalty spot, but the England internationals shot was straight at Joe Hart.
and Ronaldo rose highest on 14 minutes to head narrowly over the bar from Wayne Rooneys corner. Berbatov then drew a finger-tip save from Hart when he nodded Rafaels cross towards the far corner.
Uniteds midfield four, commanded by Darren Fletcher in the middle, were first to everything, as City struggled to win let alone keep the ball. Fletcher and Berbatov linked well to release Ronaldo before the ball broke to Patrice Evra, who couldnt keep his shot down.
Despite dominating, United almost went behind after 32 minutes. Van der Sar came a long way for a City free-kick and struggled to punch clear. With the Dutchman stranded, Stephen Ireland stabbed the ball goalwards from 18 yards. Micah Richards elected not to try and steer it in himself and in the end the ball bounced, almost in slow-motion, off the outside of the post and out for a goal kick.
It would have been cruel on the Reds, but the scare underlined how important it is to take chances in front of goal. And thats exactly what Wayne Rooney did just before the interval, when his 100th club goal his 83rd for United put United in front.
City struggled to clear a bouncing ball inside the penalty area and when it broke to Michael Carrick the midfielder lashed a left-footed shot across Hart towards the far corner. The City goalkeeper did well to keep out the shot, but could only palm the ball to Rooney, who side-footed home from two yards.
The only surprise was that it took so long for the Reds to break the deadlock. Completely dominant in the first 45 minutes, United wouldnt have been flattered had the score been 3-0 at the break.