I remember once there was a discussion once in Spain about who is a better GK, Casillas or Valdes. I was watching these two dudes on TV debating and one of them said "Its easy to say Valdes is a good coach, or that Pique is a good defender when they are rarely under pressure because Barca's attack and midfield do their job. Barca has 70+% ball posession, score goals and in doing so they relief the pressure on the deffence and GK so they look better than most defences and GKs."
When it comes to Chiellini, i would say it is the same thing. As our quality in attack has fallen drastically and our midfield couldnt keep posession even if their lives depended on it, he is put under a lot more pressure than previous years so it is only expected that his weaknesses (like passing) come out more. If your team constantly looses posession and gives the oposing team the ball, and when you are asked to play a passing game when you are a defender, its easy to look not so good.
As long as both Matuidi and Khedira play in the starting line up, we will struggle and will conceed more.