The issue is only one of the two can fit in, alongside certainties Southee and Trent Boult.
Wagner and Bracewell are contrasting in more ways than just the arm they bowl with.
Otago left-armer Wagner is an industrious operator, the sort who would bowl all day if given a chance.
Certainly he's impressed important eyes with his determination and ability to bowl long spells in trying conditions and retain hostility.
His test debut came in July 2012 against the West Indies. He's been in and out since then, not so much on form, more to do with bowling group make-up.
For example, he was unwanted in the United Arab Emirates before Christchurch when Southee and Boult, and backup from Jimmy Neesham and Corey Anderson was deemed sufficient, considering at least two spinners were needed in each test, three in the series-levelling last in Sharjah.
But his value to the side is more than just in numbers, of which the keys are 58 wickets in 16 tests at 34.48 apiece.
He is a hustler and bustler, happy to hurl in the short ball to unsettle batsmen. Certainly he can be collared, but there is an inspirational quality about Wagner at times.
Against India at Eden Park last summer, he delivered a crucial 10-over burst on the hot final afternoon as India appeared to be marching to an improbable victory.
He finished with eight wickets in the match, a timely reward considering the bouquets tend to go to one or other of the Northern Districts pair.
At Hagley Oval last week, he was at it again, churning through an 11-over unbroken spell, partly to give Southee and Boult a breather after New Zealand had enforced the follow on.
Bracewell had only himself to blame for losing his test place early last year. Late night ill-discipline had him rubbed out of the test squad, along with Jesse Ryder.
His career started like a skyrocket. Five wickets on debut to help win a test in Zimbabwe, then nine to push New Zealand to that thrilling seven-run win over Australia in Hobart, but Bracewell's bowling flattened out and his 50 wickets from 18 tests have cost 36.26.
Now back and recharged, with a couple of technical adjustments, he is the runaway leading wicket-taker in the Plunket Shield, with 23 at 13.86 for Central Districts.
McCullum knows he won't let the side down when his chance comes.
"If he does get an opportunity I'm very confident he'll perform," McCullum said yesterday.
"Doug has been bowling well for a while now and has made some great changes, and is certainly a stronger member of the squad than he was 12-18 months ago.
"But jeez, it's hard to leave Neil out when you see the heart he bowls with, and the role he takes up."
McCullum made the point that Bracewell would need to power through the same workload Wagner takes on should there be a change.
"He's going to have to operate those long spells that Neil has brought to the third-seamer role.
"But I'm confident when Doug does get an opportunity ... he will be successful for us."
Neil Wagner. Photo / Brett Phibbs
Age: 28
Tests: 16
Wickets: 58
Average: 34.5
Strike-rate: 60.6
Doug Bracewell. Photo / Paul Taylor
Age: 24
Tests: 18
Wickets: 50
Average: 36.3
Strike-rate: 63.6
