There was fuck all wrong with the substitution, some of you need to have a look at yourselves. It was hardly like he brought Martins off for a centre half, N'Zogbia is an attacking player liable to cause some different problems as well as get some width which is often crucial in the later stages when the game becomes stretched. I can see why it was Martins rather than Viduka too because Martins is more likely to give the ball away than Viduka, plus if he brought Charlie on to get some crosses in, Viduka's more likely to get on the end of them.
It was ten minutes for crying out loud!