Now that I'm thinking about it Howard was probably the senior Corps commander in the field, but Meade had enough confidence in Hancock that he had command of the units in the field after Reynolds death on the first day of Gettysburg.
Howard's set back at Chancellorsville aside, both were good generals. But Hancock's aggressive streak would certainly be a plus in this spot. Hell if he wasn't wounded he would be in better shape to participate in the Overland Campaign the next spring with Grant.
Hancock is very overlooked, but fortunately he has a major role in TL in progress.