The promising hooker, who is also contracted to the Highlanders, was forced from the field during the loss to Northland last week with a shoulder complaint.
He has been advised to spend four weeks on the sideline before a decision is made on whether he undergoes surgery.
Jesse MacDonald, who is on loan from Tasman, will start at hooker in tomorrow's Ranfurly Shield challenge against Waikato in Hamilton.
MacDonald has impressed coming off the bench after being called into the Stags set-up following a knee injury to Andrew Ready.
Ready has made a timely return to take his place on the bench for the showdown with Waikato.
The good news for the Stags is flanker Phil Halder will play in the shield challenge, despite the knee injury which forced him from the field against Otago.
James Wilson was also nursing a hamstring injury but has been named to start at centre.
"We are giving James every chance to play and he's trending in the right direction," Hewett said.
Wilson has moved out to centre after playing at first or second five-eighth throughout the first six games.
It meant Matt Johnson dropped out of the starting line-up.
"Due to other circumstances [Wilson] had to play a lot at 10 this season and that's not his preferred position, so it's good to give him an opportunity in the midfield.
"Ray Nu'u deserves a chance at second five and I'm keen to see how that combination of [first five-eighth] Broc Hooper, Ray and James goes," Hewett said
Tauasosi Tuimavave has had a frustrating few weeks, struggling with a hamstring injury as well as illness, but has returned to start at fullback on Saturday.
Rory van Vugt drops out of the 23, as does Jackson Ormond. whose brother Lewis replaces him on the wing.
Tayler Adams has returned to the playing 23 after being left out of the team which lost to Northland.
The Melbourne Rebels first five-eighth missed the first four games of the season with an ankle injury.
He started at first five-eighth against Otago but struggled, possibly because of the injury or perhaps simply due to having not enough game time.
Adams, No 8 Bill Fukofuka, and reserve halfback Nico Costa did not train yesterday afternoon as they nursed injuries.
However, the hope is they can take their place in the 23.
Hewett acknowledged few were giving Southland a shot at lifting the Ranfurly Shield tomorrow but he was comfortable with that.
He said this week the squad had embraced the opportunity to play for the Ranfurly Shield.
"A lot of people say don't mention the Shield, well the reality is it is a special occasion," Hewett said.
"The key is to not get too overawed by it, but it is an important item that New Zealand rugby teams want to win."