The trouble is that his mistakes are treated cumulatively. When he makes one, the media drag up every other mistake he's ever made and claim there's a pattern. But though his multitude of good games and world-class saves are normally mentioned and praised after the match, they are then discarded and never referred to again.
If his next match features a slip of any kind, there is absolutely no chance you'll hear anyone in the media saying 'yes, but he is the reason they won against Chelsea and he was brilliant throughout this match, so this one slip probably shouldn't affect his reputation too much.'