"Have to" is a strong and vague criterion. "Have to" in order to what? ND win all its games? ND win ten games? ND get to a BCS game? ND look like an improving top twenty team? ND looking like a well-functioning dangerous team win or lose?
I'll take the simple country road of "a well-functioning dangerous team, win or lose". If that's a goal, then:
A). one of Golson, Hendrix, Kiel;
B). two of Lombard, Prestwood, Nichols, NMartin [as I believe Hegarty will back up Cave];
C). one of Daniels, Ferguson, Brown;
That is enough to make us extremely dangerous offensively. If there's an injury, of course another next-man-in would have to step up.
D). two of Shepard, Brown, Atkinson, Jackson, Wood, Hardy, Prosise;
E). one of Councell, Spond, Niklas, Baratti.
As you see, I believe that Diaco will figure out Mike, Will, and Cat with folks like Te'o, Fox/Grace/Moore, and Shembo/Williams, but we will still be in "Wonderingland" at Dog. But with corners and a well-functioning Dog, we'll be very dangerous defensively.
F). Brindza ... consistency on KOs and FGs;
G). Special teams coach to figure out punt returns.
To complete the controversy, I'll choose Golson over Hendrix for "A"; Lombard & Prestwood for "B"; Daniels for "C", but both Ferguson and Brown to play significantly; Shepard and Brown for "D"; Councell at "E", but lots of Jamoris-style gimmicks at both Dog and Will; Brindza to settle down, not try to over-kick and get real good at it; and a "miracle upgrade" in punt returning schema, as the staff works on the theories all off-season so as not to be embarrassed in that area any more. Part of that will be a much more dangerous punt rush, which causes rushed returnable kicks.
Established players like Fox will take a real positive step up on pass defense, and Shembo will win Cat and have a smasher of a season. The left side of the line plus TEs will become a monster, and running yardage will be massive there. Who knows who will back up Cierre, but any of Riddick, GAIII, McDaniel, Mahone, Russell [both these youngsters seem VERY good; admittedly they'll have to learn the blitz blocking], and Carlisle [?], or even CamR will be WAY more effective with the new QB running threat and wider offense and Sweep-Away left side of the line.
How many wins does this transfer into? No guess --- just good and "dangerous-to-opponent-morale" level effectiveness every game.