The problem with Langfield is 3-fold.
Firstly, his confidence is at rock bottom. You can understand why, the team are not playing well, down the wrong end of the table and he is partly culpable. He isn't commanding his area or barking instructions at his defence as he should be doing. Indeed the justification for dropping Ridgers was so Langfield could talk a young defence through the game which has not been the case recently.
Secondly, when the #1 keeper is also the GK coach, we effectively have no coach. Thinking back to previous keepers we've had, I would argue that Smith, Howard, Samson and Ridgers all improved after working with Paul Mathers. I recall Celtic fans pissing themselves laughing on the first day of season 08-09 at Mark Howard's kicking, by the end of the season he was a very competent keeper. Langfield has nobody to work with, to point out where he needs to improve and what he's doing right.
Thirdly, and I think most importantly, he has no competition and less incentive to raise his game. Smith and Howard had good seasons competing against each other. Gallagher had a good season knowing that Howard was breathing down his neck. DL came in, brought in Samson but stated that Gallagher was his #1, I recall Gallagher having a relatively poor season. Samson had a good season competing against Graeme Smith, less so against Grant Adam. Having just one keeper considered for a first team place is a recipe for disaster.
We need Ridgers back ASAP. It's Rae's decision who to start, but the competition will help both he and Langfield.