Genco was Vito's consigliere but, in the book and in a deleted scene in the movie, Genco passed away. Tom took over as a temporary consigliere to Vito and remained in the post after Vito's death and through Michael's tenure as the Don (except for the events of Part III).

However, and correct me if I'm wrong guys, Michael tells Carlo that he will be his 'right hand man' when they make the move to Vegas. The term 'right hand man' does not necessarily mean consigliere, in this instance it probably means the first buffer between Michael and the caporegimes. However, we know that Michael had no intention of letting Carlo get that close - it was an act of misdirection to lull Carlo into a false sense of security, paving the way for Michael's revenge for Sonny's death.