...The quality that Michael was missing, that was pointed out to him by Hagen (in the novel) was how to say No. The Don counseled Michael that you can't say No to the people you love. But if you do, it has sound like a Yes, or you make them say no, regardless of the effort that it took. Taking that lesson and applying it to Fredo's situation might have solved the problem...
Ok, so had he not suffered this 'weakness', how could Michael have applied this lesson to Fredo's 'situation' in a way that would make Fredo feel good and yet not put the Family in jeopardy?
And, in what way would Michael foresee Fredo's situation as a 'problem'? Is there any indication that Fredo expressed unhappiness at not having more important 'stuff' to do once Michael became Don? To anyone? To Tom? To Michael? To their mother?
I go back to Turnbull's statement, which basically points out that Michael's 'weakness' had nothing to do with lack of foresight into Fredo's potential actions....but into Hyman Roth's.