A lot
http://i.imgur.com/HtH0ikR.png
He lets him get a run on him, Muller is even pointing where he wants the ball. By the time MYM has realised what's going on about him, Muller is ahead of him and has the ball at his feet.
I agree you can't blame him after that, but he had to spot/nullify the danger in the first place
Without having seen the goal there is no way you can say his positioning in that still makes him at fault for a goal that may have happened after a pass and move.
If he's even just one step forward, in line with the rest of his defence the run for Muller isnt even on. The striker starts behind him, MYM knows where he is yet still allows him to get the run right across his path and in front of him. It's pretty basic defending. Don't let your man get a run in front of you blahblah
Preach.