Williamson, who suffered the injury in Bangladesh in October, missed the drawn first Test in Dunedin but played in a lower level provincial game at the weekend and would play the second match at the Basin Reserve in Wellington.
"Kane is fine. He will play tomorrow so obviously Aaron (Redmond) will miss out," McCullum said.
Pending some last minute conversations it was likely that Williamson would not be protected in the field, McCullum said, and would likely slot back into his preferred position in the gully.
"He is pretty keen to get back into the gully. Hopefully he has no lasting pain with that thumb. I will have to have a yarn with him but I don't think he is shying away from it."
New Zealand enter the match at the Basin Reserve under more pressure than at the first game at University Oval after they had been within sight of their first victory under McCullum.
Rain during the final session of the fifth and final day left them 33 runs short of the 112 winning target with two batsmen well set to guide their side to victory.
Pundits and fans have questioned New Zealand's inability to win a Test match under McCullum's stewardship after several close calls in six draws from his 10 matches in charge in the last year, but none more so after they had snatched a draw from the jaws of victory in Dunedin.
"There was disappointment we weren't able to wrap it up, but we looked objectively and said we played some really good cricket," McCullum said.
"Whilst that's not always what's going to be discussed in the media and around the traps, you've got to be strong as a group to know you're performing pretty well and we're not far away from getting Test wins.
"If we want to be where we want to be as a team then we have to play consistent cricket like we displayed in that last game.
"Once you start doing that then good things will happen."
McCullum was already under pressure ahead of the Test series after a poor return with his own batting, which he put to rest in New Zealand's first innings of 609 for nine declared when he scored 113 to end a three-year Test century drought.
"Under pressure? You're under pressure every time you play for New Zealand," he said.
"I was under severe pressure the last time to get runs. I managed to tick that off and I guess now you can look at something else.
"You're always under pressure whenever you play for your country."
McCullum, who enforced the follow on in Dunedin before the visitors batted for 162 overs in their second innings, said the pace bowling trio of Tim Southee, Trent Boult and Neil Wagner, had their workloads managed between the matches after a three day turnaround.
While the Basin Reserve pitch was well grassed on Tuesday he was certain the hosts would not take a four-pronged pace attack into the match at the expense of promising leg-spinner Ish Sodhi.
Fourth seamer Doug Bracewell might still play, he added, but if he did it would be at the expense of one of the other pace bowlers.
"You've got to wonder if three (pace bowlers) can't get it done then whether four will," Mccullum said.
"With the all-rounder we probably have enough with three plus Corey (Anderson). We just have to work out that balance of the three."