If he's not, then it would be a reckless course of action, not something this team management, as a rule, indulges in.
The odds slightly favour Kane Williamson making the toss again.
There's no prognosis on Southee's injury to his front foot at the delivery stride. He has not been counted out of the fifth ODI in Mt Maunganui on Tuesday yet but, again, caution should be the byword.
New Zealand struggled through two tests in Australia before Christmas with Southee and Trent Boult battling niggles. They don't want a repeat in the return series next month.
Either way, it necessitates a change from the bowling attack who were left seeing stars after Tillakaratne Dilshan and co took them apart on yesterday, in reaching 277 for two.
New Zealand won the first two games easily. Sri Lanka were woeful.
Saxton Oval is a slightly slower pitch, and perhaps simply getting to new surroundings gave them a lift.
Now New Zealand must be wary of re-bolting the paddock gate smartly.
"It's always small margins I think," stand-in captain Kane Williamson said, scotching any notion that there's something suddenly badly amiss with the New Zealand team.
"I guess on the scoreboard it was somewhat one-sided. We know we need to make just small adjustments.
"One-day cricket can be fickle in its nature. It's important to address the small parts and when do that and put the opposition under pressure the game can swing quite dramatically."
Several New Zealand batsmen had been inactive in the series until Thursday due to top order class work in the first two games. It showed.
The seamers, so impressive in Christchurch, were off their game, only left armer Mitchell McClenaghan avoiding a real pasting.
There could be a place for leg spinner Ish Sodhi, on the basis the pitch will be more helpful to spinners after some wear.
Matt Henry returns for Southee, and after his eight wickets in the first two games, should be back. The pre-planned move to leave him out after the first two ODIs looked peculiar after Henry's fine performances in Christchurch.
Gunathilaka won man of the match for his sparkling hitting; Dilshan showed his class, while Lahiru Thirimanne's unbeaten 87 suggested he has recovered his touch after a pair of ones in Christchurch.
Sri Lanka looked much more up for the contest yesterday. Whoever said the inspiring words before the start, keep him on.
"If we go to negative thoughts, well, we can't (win)," Gunathilaka said.
"This New Zealand side is a very strong side, they have a very good bowling attack so we have to be positive."
Speaking of Williamson at the toss, for all the flak McCullum receives for what is actually a reasonable record at the coin flip, Williamson is likely to get some gyp from his team given his success rate.
His win after yesterday was just his fifth success in 16 ODIs.
