I believe Phil Sheridan could have stepped into the void if Grant had not been able to take command of the army. After all they thought along the same lines. Sheridan maybe a little more severely than Grant. Judging by Sheridan's tactics in the Shenandoah Valley in 1864 he could have been just as tenacious. The war abounds with What Ifs. What if McPherson had not died at Atlanta? Would he have made the March to the Sea instead of Sherman? Could he have done as well as Grant? His fellow officers seemed to think he had all the necessary attributes for supreme command.