I've already shown you the scan where obito States Danzo's Eye is ready, and the scan where danzo said (to himself), that he didn't want to use the genjutsu on Sasuke but save it for "Madara".
Danzo has a big time Gap between uncovering his Eye to getting stabbed through karin, Easily enough time to cast KA.
Sasuke did noot attack befor he could use it, Since he never really planned on using it on sasuke because of Obito being there.
As i said, he gave sasuke a little speach befor sasuke stabbed with chidori spear, he could of used it if he did not have "Madara" to fight afterwards.
IF Obito wasn't there watching over the fight, Danzo would of KA'ed sasuke, Fact.