He's not half the player he was in his debut season at Barca. Reasons could be few for him to have his confidince diminished:
1) He was overlooked by German NT coaches for the starting spot for so long, the confidence in the qualities of Neuer were never under any suspicion, even in injury periods. I don't know about German public point of view, but we have few local users here who might share some insight.
2) The three disaster CL games in 4 years Barca experienced with MATS in goal. It's called PTSD.
3) The lack of competition since the Claudio Bravo days. No matter the number of goals conceded his place is cemented. With all due respect Cillessen and Neto are not the Bravo level and the backup role they had/have at Barca isn't helping. "Okay, now you play in Banana cup, Copa Catalunya, make us proud!". Lucho balanced this better with one GK playing in CL, other in La Liga, until he had Bravo in disposal, of course.
4) That Falcao goal was absolute Busi/Pique disaster show and MATS stood there like those training free kick mannequins. 4) is not really a factor in his decline, I just wanted to share my thoughts on that goal.
MATS's current level is comparable to another mediocre, who plays at Juventus - Szczesny.