''That career has passed me. It's BJ's opportunity and he kept really well in the warmup on a tough surface.
''He's looking forward to it, he's earned his chance and I'm sure he'll do well."
Watling's last test as wicketkeeper produced a maiden century against Zimbabwe at Napier in January. He then suffered a hip injury which forced him out on the eve of the opening South African test at Dunedin last season.
Kruger van Wyk got his chance and kept for the next nine tests before being dropped for this tour.
Watling's form in the shorter game has been good, and he will bat in an important position, one spot ahead of the seasoned James Franklin.
''It's a pretty good top five and hopefully we can find some ways to get partnerships going and put pressure back on South Africa," McCullum added.
The one sticking point for coach Mike Hesson and McCullum is completing the seam bowling trio.
Only one spinner, probably Wellington offspinner Jeetan Patel, will be picked unless there's strong evidence of a fast-drying surface in the two days before the test starts.
Trent Boult and Doug Bracewell will play, leaving veteran Chris Martin and left armer Neil Wagner vying for a place. The fifth fast-medium bowler in the squad, Mitchell McClenaghan, is at the back of that queue, McCullum confirmed.
He was the late selection to replace Southee, and McCullum praised his form in the T20 series which preceded the tests.
''He was probably the find of the series," McCullum said of the strongly-built Aucklander.
''I'm sure he's learning a lot about the dynamics of the long game in this setup but at this point he's further back in the pack than the other seamers."
The game against the South African Invitation XI was called off in mid-afternoon on the final day, with New Zealand 117 for two in their second innings of a game in which the tourists' used 13 players.
Guptill hit an unbeaten 68 off 79 balls, McCullum made 28 but Brownlie, sidelined by a stomach bug late last week, had only a short time in the middle, getting six.
McCullum is confident he'l l be ready come Wednesday.
The bowlers all got through a decent amount of work in Paarl, leaving McCullum confident they will be primed for the test.
''It was important to get their bowling loads up. They would have liked more response from the wicket but they're pretty happy and are going into this game with as much preparation as they could possibly have had."
He is happy with the balance of the side for the first test; ''we just have to make sure we select the right person for that last role."
That will in large part come down to their reading of the pitch.
South African batsman AB de Villiers today predicted the seamers will enjoy the first two days, the batsmen could prosper on the third before spinners will get a look in over the last two days.