He was poor but then so were others. Only an idiot would put this result all on him but I've said it in the past as well, Cesc is not as good when you deny him time and space. You don't get that luxury against better teams like Madrid etc. He also loses possession and gets hassled off the ball a lot easier than our other players when you apply pressure on him.
The biggest problem I have is that he usually occupies a position where I would rather see Iniesta play. Iniesta along with Messi are two of the best players in the world in the final third and I don't think that they should be so far apart from each other.