"We just felt it was a good chance to try and freshen him up this week. Hopefully we still get it right and then he'll be a little bit fresher leading into the last round and the playoffs."
Indeed, Retallick played the majority of the Welsh series, racking up 211 minutes across the three tests. And although Cane also started each match, totalling 197 minutes in the series, Rennie felt a couple of days off training earlier in the week would suffice for the co-captain.
The coach also believed Cane was ready to add another string to his bow. With Tom Sanders, who started at No 8 last week, moving to No 6 as Taleni Seu shifts to lock, Cane has been entrusted to fill a hole at the back of the scrum.
Super Rugby rookie Lachlan Boshier rounds out the loose forwards while the returning Liam Messam will begin his Chiefs comeback from the bench.
"We've talked to Sam about playing 8 a lot, and he's finished a couple of games at 8 for us," Rennie said. "We've been really happy with Lachie, and having two sevens on the field from a post-tackle point of view, we think will be really good for us against a Reds side who are very strong in that area."
In the backline, All Black halves pairing Tawera Kerr-Barlow and Aaron Cruden are set to return to the starting side after being eased back via the bench in last week's crucial win over the Crusaders.
And Anton Lienert-Brown will again play alongside Seta Tamanivalu in the midfield, with Rennie conceding that concussion had likely curtailed Charlie Ngatai's outstanding season.
"Realistically, it's going to take him two or three weeks to get up to speed from a conditioning point of view, and even if he stops having symptoms, we'd probably want to give it another week before we amp things up.
"We're not saying he's definitely gone for the season but the chances are pretty slim."