They beat the Highlanders 28-7 in Hamilton on Friday, but McMillan said afterwards there was evidence of some tiredness in their game.
"It's been a big pre-season, post-Christmas – a big game down at Christchurch, on the road over to some tough conditions and a good Moana side [in the Super round in Melbourne], and then on a short turnaround back here.
"A good majority of our squad have played big minutes over those three games, so there's probably a need to freshen some guys up next week."
The Highlanders had provided an arm wrestle, and he was pleased with the side's defensive effort. No8 Luke Jacobson had been outstanding in that department.
"He led the charge, got off the line and got some big shoulders on and to restrict them to one try is always pretty pleasing. We had some good moments on attack, but were a bit clunky and probably left some points out there."
The Chiefs play the Rebels in Hamilton on Saturday.
"We've got some depth in the squad, and there have been some disappointed guys over the last couple of weeks who have been itching for their opportunity. So, it's an opportune time to inject them and give them an opportunity to put their best foot forward."
At the same time, he was conscious of the challenge the Rebels would provide after their efforts against the Hurricanes and Waratahs in the last two weeks.
Highlanders coach Clarke Dermody said the side continued to improve, but they still needed to be competitive against the best teams.
"We showed glimpses of what we could do, but not for the whole game. Against the Chiefs, the Crusaders and the Blues, you have to do it for 80 minutes. We're not quite capable of doing that yet."
However, they faced a run of games where they hoped to play better, starting with the Force in Invercargill this week.
The Highlanders' injury woes continued with lock Pari Pari Parkinson suffering concussion, while first five-eighths' Mitch Hunt and Freddie Burns suffered shoulder and knee injuries, respectively.
Just how bad their injuries were would be known during the week.