He may have gone backwards, but I guess a lot of it has to do with us being more attacking. Lets face it, he claimed golden gloves in Mancini's years were our midfield and defense shielded him a lot. Now that we are far more expensive, his mistakes are more prominent.
What makes it more interesting is that Mancini wanted to replace him in the year he won the golden glove. The reason, our style of play then stopped the opposition from scoring but when Joe needed to step up he made mistakes, He had great games too, the one v Dortmund where he single-handedly kept us from defeat. But why would Mancini want to replace him? Because when expected to perform he didn't.
Its the same case now, except the Joe doesn't have the same shielding as before. Now he will make some awesome saves (Everton, CSKA away) but he would give goals he really shouldn't. Stoke being an example, Sterling away last season at the candle lighters, he should have been back in his goal. He also has a pronounced difficulty on his near post, and I think its being targeted. Compare his performance against stoke to that of Courtois against Leicester. He saved two goals at the very least after the defence was breached, probably saved 2 or even 3 points. That was what is needed of a GK in a top team, saving points consistently when your defense fails. Joe does make the spectacular save but he misses too often when he shouldn't. I think this season is make or break for him unfortunately.