I'm not sure how popular this will be, and I didn't do it specifically to dig him out I promise, but I thought I'd go through the games because I was a bit unsure about how many errors there had been, via a combo of memory/match reports/video/ratings threads, and it's not great reading this season. Obvious errors I'd say have been:
Chelsea (H) - Late winning goal beats him from a ridiculous angle.
Liverpool (H) - Through ball that should be claimed, in a previous life he'd have been on it without a thought, instead rooted to his line and goal is scored.
Arsenal (A) x2 - Personally thought he was slow and/or made to look silly on both goals. There's at least one error here.
Stoke (H) - Gifts them a late consolation goal flapping at a corner.
Southampton (H) x2 - Errors for both goals. Fluffs saving a shot, and flaps at a corner.
Chelsea (A) - Somehow completely misjudges a cross.
Stoke (A) - Makes a hash of sweeping up.
I'd make that 8 or 9 errors for goals. The official stats if anything are being a bit nice to him.
And there are other games such as Dortmund (A), Man Utd (A), Liverpool (A), Juventus (H) and maybe even WBA (H) and West Ham (H) where he's either made errors unpunished (Utd) or people have criticised some of what he has done eg. going to ground too early and leaving too much of the goal to aim at, and they probably have a point in some cases, but I felt were too subjective to include.
Also note the number of 'big' games there, something that was starting to be noted last season as well (for example the two howlers against Man City).
This post started just purely to try and tally up this season for my own benefit, and of course it doesn't mention the many good things he will have done over the course of a season, but I don't think it's unfair for him to be criticised based on looking back at how his season has gone. These haven't just been mistakes in one area either, there's been shot-stopping ones, aerial mistakes, sweeping up mistakes, etc, and silly ones at that. I think it's hard to argue that there hasn't been a little drop in standards and you do expect a bit more from him given his class, and that he is the captain, senior player, and should be leading from the front - especially on the biggest occasions.
I've long had a slight concern that he isn't a big match player which is surprising considering his experience. This might date back to his error for Sanchez's goal in the NLD that gifted 10 man arsenal a point after Harry's wonder strike.
If might just be a simple case of goalkeeping errors being more costly but I was livid with him for that one (probably in part due to the quality of Harry's goal deserving to win the NLD).
This season it is Chelsea that we are up against as direct rivals and he has gifted them two goals.
It's probably nothing just coincidence and I don't think there's anyone better we could get plus Poch values him highly but hopefully he's working hard to improve as we are going to have more and more big games over the coming years.