Sergio Aguero bagged the opener for Manchester City in the 16th minute thanks in part to a stinging cross from Alexander Kolarov to give the home fans the early boost.
Samir Nasri won the ball and advanced down the left side, ending up just outside the box. Manchester United defender Chris Smalling closed him down well, but Kolarov supported with a great run just behind Nasri, having lost his defender Valencia.
The French winger flicked Kolarov the ball, and he lashed in a perfect cross which Aguero connected to perfectly with a 90 degree touch-volley into the top right corner.
The loss of Robin van Persie for Manchester United now looms even larger with the visitors playing from behind early.