Oh man.... again
Chiellini wasn't marking Drogba when he scored, he was simply inside when Kalou assisted Drogba which lead to the goal. He takes the blame, everyone was off except him he was still inside. Check the goal again please.
What do you mean he had to cover for Molinaro? That's his job man, Molinaro was running his ass of that's Chiellini and Legro or any other CB to cover their flanks at times. He failed and it's not an excuse, seriously.