Kander is a top notch recruit for this office. He probably loses by high single to low double digits though. But it looks better to oppose a potentially vulnerable Senator in a wave with someone competent than leaving the seat to be fought for by some dude.
I'm fairly confident even this early that this will be a single digit race. People really overestimate just how red is Missouri is. Had Obama contested the state, he probably would've cut the margin by a few points. Hillary plays better than Obama there, and there were tons of Romney/Nixon/McCaskill voters anyway. Nevertheless, Blunt is obviously the favorite for now.