I watched the game again last night too, after the double sub, we spent the first 5 minutes climbing all over united, and had 2 very good chances, before Rooney's run through the centre (not down our left), where nobody put in a proper challenge. It was this that caused the nervous last 15 minutes for me, we suddenly realised the game wasn't over, and started defending desperately rather than cleverly, as we had until then.
Dzeko, whilst he does defend well at corners, was brought on to defend from the front by holding the ball up, and bringing others into it, its a pity he wasn't interested in doing that, I think he managed it once.
Unfortunately we resorted to panic defending, brought on I guess by our recent poor form, rather than using our brains and the space on the pitch to run time down, instead giving the ball back to united far too easily, when we should have keeping possession.