Williams has made four appearances for Canterbury at second five-eighth, including a starring role off the bench in last Saturday's 37-30 win against Wellington, but will move out a position against in one of four changes to Canterbury's starting 15.
Penney said Fruean, who had played eight matches in a row this season, was due a spell on the reserves bench.
"Robbie has played every minute of our last seven games and we are managing his workload as we do all our players," Penney said.
"Robbie has been one of our most consistent performers and we're really happy with his form and attitude.
"We're confident Sonny will go well at centre given his skills and the skills of those players around him. It's an exciting prospect."
Hooker Corey Flynn (sternum) and prop Peter Borlase (shoulder) are unavailable and have been replaced by Steve Fualau and Andrew Olorenshaw, respectively, while a minor knee injury has ruled out Stephen Brett. He is replaced on the reserves bench by Sam Monaghan.
Paul Ngauamo and Rodney Ah You move on to the reserves bench as front row cover.
The Otago team is: Chris Small, Joe Hill, Josh Tatupu, Glenn Dickson, Fetu'u Vainikolo, Chris Noakes, Sean Romans, Charlie O'Connell, Alando Soakai (captain), Hoani Matenga, Hayden Triggs,
Josh Townsend, Halani Aulika, Peter Mirrielees,
Kees Meeuws.
Reserves: Wyatt McKay, Sam Hibbard, Paul Grant, Brad Cameron, Johnny Legg, Paula Kinikinilau, Ryan Shortland.
