Keep in mind when Tom is first introduced Michael tells Kay he is going to be consigliere, and he goes out of his way to mention that Tom is not a Sicilian. This shows Michael's less than high regard for Tom even before he gets into the family business.
good point DT... but I often wonder this... how/when is Tom consigliere??? We just take for granted that this is true, but I, honestly, never really bought that he was ever anything more than an "interim" consigliere. (and this may be a whole nother topic in and of itself, so I'll create ;-) )