Koroi (19) was scratched from the team after a check by medical staff and it should be confirmed over the weekend whether he has mumps.
His absence has forced changes to the Otago backline, with Fletcher Smith, who was set to come off the bench, starting the game at fullback.
Young University centre Josh Timu comes into the reserves and will make his first class debut if he gets on.
There were not many options left at fullback, as Michael Collins and Matt Faddes are both injured.
Also in the back three, winger Mitchell Scott is battling various niggling injuries and will not play. He has been replaced by Leroy van Dam, who will make his starting debut for Otago.
At halfback, Josh Renton comes into the side, swapping places with Jonathan Ruru.
Renton came on last week early in the second half and quickened the game up with his pass. Renton is more a passer and a link man, while Ruru is more combative and it is a good mix for Otago to have in its number nines.
In the front row, loosehead prop Craig Millar gets his first start, replacing Aki Seiuli in a case of rotating players.
Lock Blair Tweed has recovered from a virus and is named on the bench. New import Latu Vaeno, a winger on loan from Taranaki, is on the bench.
Mumps is contagious, so Otago coach Cory Brown will be hopeful it does not spread through the team.