He will also be wary of anointing the next captain by having Williamson, most people's pick to eventually take over, too early. It happened to McCullum himself when he was Daniel Vettori's wingman, only to lose that role in strange circumstances.
McCullum has a good point in wanting Williamson to settle into his role in the side without the burden of expectation, but in this case he needs to develop a sense of PR guile.
Southee would perhaps be the most inappropriate vice-captain he could choose.
Let's start from the top.
McCullum is managed by Stephen Fleming. Tim Southee is managed by ... Stephen Fleming. There was already misguided flak flying around about Fleming being the New Zealand Cricket Players' Association representative on the panel that appointed Mike Hesson as coach; Southee as v-c provides the ammunition for a full-blown, grassy knoll-type conspiracy.
The phrase "not a good look" is too often trotted out but in this case you could get away with it.
Already red flags will be running up the pole at New Zealand Cricket. An impeccable source said while it was true that McCullum presented professionally and persuasively when he was forced into a captaincy run-off with Ross Taylor, the panel had a major problem with his desire to promote Southee into a leadership role.
NZC couldn't see it then and, no matter how well the seamer has done to get his bowling back on track, probably can't see it now.
Southee has a reputation for being one of the lads and, even allowing for the fact that people grow up, if there's one tag you'd never apply to him, it's leader of men. Perhaps he's different inside the group, but perception can be a hard thing to shake.
Take his batting. Southee blazing 156 this week for Northern Districts just highlights how badly he has underachieved with the willow since scoring 77 not out in his first test.
Try looking your teammates in the eye and telling them what they should be doing when half an hour earlier you've chipped one down the throat of the only fielder on the boundary.
Taylor and particularly McCullum are not immune when it comes to rash dismissals, but they have demonstrated they can fight for runs.
This also fails to give credit to Southee for change. He might have now realised just how good he can be, just how hard he needs to work and just how much he has to offer this side.
In a few years he might make a fine vice-captain, but right now he comes with a flashing warning sign.
Williamson would be a smarter choice; so too would be, when he returns, Taylor.