Aurier's mistake (ended in De Bruyne's shoot from outside the box, at minute 50) came immediately after a very good defensive action - Torres tried to get in behind him, Aurier spotted the run and recovered well; there was still some space left for a cross, so he blocked that space without actually touching the ball (or Torres). It was an energetic and intelligent move... But then he got the ball seconds after - without any time to stop and think about what to do next, in a safe manner - and his brain let him (us) down.
I think it was a very symbolic mistake. Aurier shouldn't be involved in "consecutive actions". He shouldn't play three games in the same week, for example. He needs to be well prepared for every time he's on the pitch for us. And he is a good player if used correctly.
By the way: Aurier played really well against Chelsea, United and City. Say what you want about him, but Aurier is a player with a soul.