Morons, as much as we like to blame him, it was clearly Chiellinis error.
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It's really not.
Who is Bonucci defending? Before the cross comes in the goalscorer is right next to him, instead he switches off moves forward and leaves Chiellini and Alex Sandro scrambling inside to try to defend an attacker further inside than they needed to.
If Bonucci defends it properly he stays on the goalscorer, Chiellini stays on his man and Alex Sandro defends the man attacking the back post. Would be simple.
I'd add that our 'keeper was caught flat footed.