Manchester United have already taken a lead on a fantastic move, though David Luiz will get credit for an own goal. It's harsh on the Brazilian defender, who couldn't have possibly known much about it, and the goal was really due to a brilliant strike by Robin van Persie.
While David Luiz didn't do anything wrong on the actual own goal, the initial defending by the Chelsea back line was very poor. Ashley Young was given a ton of space to run into on a through ball, though he didn't do a great job taking it. Instead, he laid off to van Persie, who unleashed an excellent strike to the near post from around 25 yards. The ball beat Petr Cech and ricochet off the post, then deflected off the back of David Luiz before hitting the back of the net.
Chelsea had the first half-chance of the game in the first 90 seconds, but the goal actually came on van Persie's second shot from long range. He might have just felt confident in his abilities early, but it wouldn't be shocking if this was a tactic of some kind from Alex Ferguson and his staff.