Langfield had 2 or 3 alone against St Mirren and several more against United. Whether or not another keeper, mediocre or otherwise, would have made them is pretty irrelevant as we won't know. The fact is Langfield did and it would be difficult to find goals Langfield has been responsible for recently.
I'm saying this from the perspective of someone who would be happier with someone else in the long term ie next season starting as No.1. Him and the defence aren't doing at all badly as they are.
But when there's mistakes made (and there will be) the "I told you he wis shite" folks are always ready. There have been plenty made right in front of Langfield that have resulted in goals in recent weeks.