He wasn't aware of Danzo's presence and didn't know the mechanics of Izanagi. Danzo was behind him. If Sasuke could see the attack coming, Karin wouldn't have even warned him. Yet she was yelling hard to make sure Sasuke heard her...
If one's fighting style relies heavily on sneak and surprise attacks, then using them and winning with them is something that makes you believe that one is weaker than their enemy? Ok. Let me not comment on that.
At that point, he was very well aware that Danzo would reappear after being killed.
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So he knew enough to expect a surprise attack, and someone warning someone else doesn't mean that the person didn't see it coming, or that the person wouldn't be able to react accordingly on their own. Karin saw that Sasuke's back was turned, how would she know if Sasuke would be able to dodge on his own or not? From her point of view, it looks like he's about to get hit, thus she warned him.
Like I said. If Danzo can only kill Sasuke if his back is turned, then he isn't stronger. If Konohamaru stronger than Naruto if he sneaks up on him and stabs him in the back? Lol, obviously not.
Not to mention Danzo only got that opening because Sasuke used Amaterasu and Susanoo at once. If he had Complete Susanoo from the start, he'd be using arrows instead, thus no opening for Danzo. So once again, Sasuke>Danzo.
To me, Sasuke broke free from CS thanks to special chakra sprouting in his brain - something that upgraded his Susano'o after he had been overflown with emotions. If we have Complete Susano'o Sasuke vs Danzo from the beginning, there's no chakra sprouting and no getting rid of CS.
Well, that's just an assumption, backed by no fact. The special chakra in his brain was only stated to be related to the awakening of the Sharingan and nothing else. There isn't even evidence that the special chakra in the brain is related to Susanoo, or that Susanoo is formed from that.
From what Obito said and what the manga shows. Complete Susanoo was enough of a surge to allow Sasuke to break out. That's it.
Also, using Complete Susano'o drains obviously way more chakra than using Skele Susano'o. There's no way Sasuke could outlast Danzo with it, since it doesn't matter whether which Susano'o attacks he survives.
Except Susanoo doesn't need to active all the time, as the manga showed, Sasuke was capable of fighting by using it intervals. Not to mention Amaterasu+Susanoo drains a lot of chakra, more than Complete Susanoo (Not in the long run) as Susanoo dropped aftwards, but Sasuke was forced to use both at the same time since he lacked a consistent way to pressure Danzo, something he gains with Complete Susanoo.