There is no way Grant could have pulled off organizing the invasion of Europe in WW2.
Why not?
Maybe not, but Eisenhower would not qualify as general of the United States Armies since the duty was split with Ike in the European theater and MacArthur in the Pacific. "Supreme Allied Commander" is still a pretty bad assed title.
No individual could without a reliance on expert staff work. Grant had that. Several of his contemporaries relied on a more ad hoc approach.