Championed by his club coach Justin Lock and from a strong South Taranaki rugby family, the aggressive Middleton has the faith of the Wanganui staff that he can fill some giant boots, even if it has come a little earlier than intended.
"There's no worries about that," said Caskey.
The late switch highlights what a tough season this could well be for Wanganui in their quest to become the first Heartland union to three-peat with the Meads Cup.
Of those players either selected or bracketed from the last time these teams met - Wanganui's impressive 58-26 win in the 2016 Meads Cup semifinal - only ten remain, of which only four were in the starting lineup.
First-five Craig Clare is the only surviving player from the six tryscorers that day.
While imports, periphery players and youngsters leaving each season is nothing new, the gradual departure of so many local veterans could well add to the nerves of the 2017 lineup under new skipper Roman Tutauha, back in the starting XV for Cole Baldwin after getting over last week's cold.
"There's probably a bit of a mix with the excitment too," said Caskey.
"It's a bit of a different set up, and different era, a new captain.
"Everyone's pretty apprehensive about that first game too - you find out where you're at compared to where you think you're at.
"It would be nice to play a team like East Coast for a warmup, but not to be.
"If you're going to win the competition, you've got to beat anyone."
For outside back cover, Wanganui only has Samu Kubunavanua on the bench, however Caskey is confident they can work with other combinations as centre Kaveni Dabenaise and Auckland import fullback Jona Sawailau could move positions.
"Craig [Clare] can play anywhere from second-five to centre as well. We've got options to cover."
Wairarapa-Bush were beaten 33-17 under lights by Manawatu Development in their final preseason match last week, but 71-test All Black Piri Weepu showed all the old skills with his passing, decision making and defence - playing halfback in the opening stanza and first-five in the next.
The great character Weepu has had a tough time since leaving New Zealand in 2014, playing only 32 games for his four English and French clubs, but the 33-year-old said he was looking to have a bit of fun before he hangs up the boots.
Wanganui have successfully faced former All Blacks before as Rico Gear fronts periodically for Poverty Bay while serving as the union's development officer, while Zac Guilford turned out for Wairarapa-Bush last season.
But this year could well bring added national interest to Heartland rugby as Weepu is one of four test players who will appear this season, joined by Alby Mathewson (King Country), Regan King (Mid Canterbury) and Corey Flynn (West Coast).
"I think it's great to see a few of them prepared to go back and give a little bit back, it's brilliant," said Caskey.
"They're not at the peak of their powers, but people will want to see them."
Focusing on the Weepu show would be to underestimate the rest of a Wairarapa-Bush outfit who have been repeat Meads Cup semifinalists.
First-five Tim Priest will combine with Weepu, while fullback Sam Monaghan, in-form No8 Chris Raymond and flanker and captain Eddie Cranston are all to be treated with respect.
Kickoff is 2.30pm, after the curtain raiser between the two union's Development XV's.
The teams are:
Wanganui
1. Hamish Mellow; 2. Roman Tutauha (c); 3. Viki Tofa; 4. Sokonaia Kalou; 5. Sam Madams; 6. Fraser Hammond; 7. Jamie Hughes; 8. Bryn Hudson; 9. Lindsay Horrocks; 10. Craig Clare; 11. Bruno Tuivai; 12. Timoci Seruwalu; 13. Kaveni Dabenaise; 14. Simon Dibben; 15. Jona Sawailau.
Reserves - Forwards: 16. Cole Baldwin; 17. Gabriel Hakaraia; 18. Tremaine Gilbert; 19. Angus Middleton or Campbell Hart. Backs: 20. Tom Stewart; 21. Dane Whale; 22. Samu Kubunavanua.
Wairarapa Bush
1. Nick Hohepa; 2. James Pakoti; 3. Jacko Hull; 4. Regan Pope; 5. Andrew McLean; 6. Brendan Campbell; 7. Eddie Cranston (C); 8. Chris Raymond; 9. Piri Weepu; 10. Tim Priest; 11. Nikora Ewe; 12. Cyrus Baker; 13. Matt McCrea; 14. Soli Malatai; 15. Sam Monaghan.
Reserves - 16. Hone Vella or Tufui Sosea; 17. Jayden Mason; 18. Lachie McFadzean; 19. Chaz Matthews; 20. Katia Inia; 21. Tipene Haira; 22. Corey McFadzean.
Wanganui Toyota Development
Wiremu Cottrell, Jack Yarrall, Marek Willis, Joseph Edwards, Keanu Puki-Kaa, Josh Lane, Alex Mulipola, Jackson Campbell, Kieran Hussey, Taione Ratu, Cade Robinson, Lake Ah-Chong, Kahl Elers-Green, Josh Fifita, Grayson Tihema, Josaia Bogaleka, Osea Tarogi, Elijah Ah Chong, John Kilisi, Karl Pascoe, Te Rangitapu McLeod.