Apparently neither Puzo nor FFC saw any need to introduce an underboss. In fact, FFC also eschewed a consigliere in II probably for the same reason. Those two positions just did not serve any purpose in the films' narrative. As a further example, it apparently did not serve Puzo's purpose to assign the Corleones more than two capos longterm.
I had always chalked up the no Consigliere in II to the fact that it gave Michael an illusion that his end of the family was legitimate or going legitimate.
