Basically, I agree with DT. However, Michael also blamed Tom for not protecting the family: Sonny's murder and Vito's near murder. That's why Tom was not a wartime Consigliere.

As DT points out, Michael tells Kay at the wedding, Tom will become Consigliere although he's not Sicilian (he also unnecessarily tells Kay Tom's last name although that may be a way for Michael to inform the audience about how Tom became a family member). In II Michael tells Ola that Tom only handles certain parts of the family's business. Later in II Michael reveals Tom's having a mistress. Of course, the flashback scene says it all: "You talked to my father about my future ... my future."