Just don't expect Ngatai to respond with the histrionics that might seem to suit the occasion.
"Anyone who knows Charlie knows he's very understated," Rennie told Radio Sport. "He is incredibly excited - for him - but Joe Bloggs might not recognise that.
"He's the type of guy who, when he's had an absolutely barnstorming performance, you ask how he went and he'll say, 'Oh, not too bad'. He's not that excitable but I've certainly seen it about him [this week] - he's got a bounce in his step."
That's certainly understandable considering Ngatai, at various points during his slow rehabilitation, thought about quitting rugby altogether. That would have been a cruel end to a promising career - and one fellow one-test All Black James Broadhurst experienced last week - but the concussion cloud last month finally lifted enough to enable his return.
Three games for Hamilton's University club later and Ngatai will provide a big boost to the Chiefs midfield, with Rennie holding no doubts about including him in the starting XV.
"He's ready to go," Rennie said. "He's keen to start, get into the warm-up and get a feel for the game. He's got a fair bit of footy under his belt now - two 80-minute cracks and a half a game. We're really excited."
That emotion extends beyond the inclusion of Ngatai, who's not the only long-term absentee set to make his season debut. Nathan Harris has been stuck on the sidelines since rupturing his ACL on All Blacks duty last August, but the hooker will provide cover for Hika Elliot on the bench against the Reds.
"It's really cool and great to have him back," Rennie said. "Obviously he provides a bit of size and experience, so we're pretty excited for Nathan as well."
In addition to that pair, the Chiefs will be further strengthened by the recall of James Lowe, Tawera Kerr-Barlow, Michael Leitch and Brodie Retallick, a first-choice quartet rested from last week's unconvincing win over the Sunwolves.
And with Ngatai finally allowed to again link with Anton Lienert-Brown in midfield, Rennie was hoping his attack would rediscover the spark that has been missing in recent weeks.
"It's just about as good a backline as we can put on the park," the coach said. "We know we can be a lot sharper, especially around our attack. With a few fresh legs coming back in and a lot of enthusiasm, this is the time to start building."
Chiefs team to play the Reds on Saturday night:
1. Kane Hames
2. Hika Elliot
3. Nepo Laulala
4. Dominic Bird
5. Brodie Retallick
6. Mitchell Brown
7. Sam Cane (c)
8. Michael Leitch
9. Tawera Kerr-Barlow
10. Aaron Cruden (c)
11. James Lowe
12. Charlie Ngatai
13. Anton Lienert-Brown
14. Toni Pulu
15. Damian McKenzie
Reserves: Nathan Harris, Atu Moli, Siegfried Fisi'ihoi, Taleni Seu, Liam Messam, Jonathan Taumateine, Sam McNicol, Shaun Stevenson
