Kenpachi is over rated at this point. He'll come into the match cocky and Itachi will finish him. Genjutsu out classes Tosen's abilities, and while he's in it he can be killed by an array of things from Itachi.
Hopefully that'll change....of the recent chapters of him with his "training" with Unohana and the cliff hanger it left off of....
I think Kenpachi when he is revealed again will probably have a total different mind set.
As for the fight if we're going by Kenpachi pre-time skip I can see Itachi ending him with Totsuka but thats a big iffy.
But of his recent showings post time skip if he has no interest in an individual he is fighting he will kill them rather quickly.
Lastly Kenpachi isn't stupid....he may not be at Itachi's level of intelligence but isn't an oaf.
I would like to know the "array" of things Itachi could off Kenpachi with cause I only see one option.