The Price of Potential
He has not seen the infamous Sasuke Uchiha that everyone has been clamoring about ever since news leaked that his own death was imminent. Had he seen him, he would have certainly been underwhelmed: the boy two years his junior was a whole head shorter than him, had a rather girlish face, and lacked even a hint of musculature. The all-important supposed genius vessel-to-be for Orochimaru-sama was, for all appearences, just a prepubescent child. But Kimimaro has not seen Sasuke Uchiha, and all he knows about his soon-to-be successor is that the boy is young and that he can give Orochimaru-sama what he cannot: time.
He has not seen the Sasuke Uchiha that Kabuto says can act as Juugo's new container. Had he seen him, he would have been unequivocally insulted. The preteen lacked both calmness and the ability to literally act as a physical restraint for the origin of the Cursed Mark. At the very least, the kid even lacked the finesse and charisma to charm Juugo into sanity or the brute strength to bow him into submission. The Sharingan user was simply a gennin with a few parlor tricks that, against Juugo, would yield his death. But Kimimaro has not seen Sasuke Uchiha, and all he knows about his replacement is that has eyes fit for both a killing machine and a hypnotist.
He has not seen the Sasuke Uchiha that Tayuya heard was handsome albeit stoic. Had he seen him, he may have chuckled. He would have recognized the indifferent exterior as phony with tumultuous emotions stirring beneath. He would have correctly identified the eyes girls called 'dark and mysterious' or 'menacing and sinister' as really just being full of pain and unwavering resolution. The rumored rookie heartthrob from Fire Country was really nothing more than the average dysfunctional youth, undeserving of attention and affections from the likes of Karin, who liked to go on and on about people's looks and chakra. But Kimimaro has not seen Sasuke Uchiha, and all he knows about his substitute is that he has the temperament of an obstinate dog.
No, Kimimaro has not seen the newcomer from the Hidden Leaf village, but he has heard enough to know that above all things, what Sasuke represents is potential and only that. He is inexperienced, unskilled, and lacks appropriate dispositon. He pales in comparison to Orochimaru's very first preferred vessel candidate; the boy's own older brother. And while Kimimaro has been regarded as a broken tool and set aside to rust, this 'Sasuke' is considered an incomplete tool and will be whittled and molded into something exquisite and lethal. Where Kimimaro was once prized for what he is, Sasuke is valued for what he could be.
He is past and Sasuke is future.
Kimimaro does not need to see Sasuke Uchiha. He is unsure if mere potential is worth this final sacrifice - if this one child can be his absolute replacement - alas, he will do this, if only to be useful.