Steven Luatua's importance to the Blues has become even more important. Like last week he will start at loose forward before moving to the second row after halftime, but injuries could quickly change that plan.
The case of Donnelly, who strained a calf warming up during the 30-12 win over the Highlanders at Eden Park last weekend, is a perfect example of that.
Kirwan announced his team early this morning, with Jordan Manihera covering lock on the reserves. Manihera, a loose forward, will be joined on the bench by captain and flanker Luke Braid, who will be rested after a heavy workload this season.
"It's the competition isn't it," Kirwan said of the injury toll. "It's something you need to prepare for and something we spoke about after last year. Part of the reason for playing a bigger squad and giving guys opportunities was so you've got better cover for these moments."
Jerome Kaino will captain the team on Friday, with Kirwan determined to put Braid in cotton wool. The openside flanker, who has played all but six minutes of the Blues' six matches, received a "stinger" shoulder injury against the Highlanders and will not do any contact training this week.
The locking hope on Kirwan's horizon is wider training group player Tom Murday, who will return from a knee injury to play club rugby this weekend. Francis Saili's return is also welcome - the All Blacks midfielder has missed five matches after aggravating a foot stress fracture during the opening defeat against the Highlanders. "For me it's starting from the bottom and seeing what I can do from there," Saili said.
"The Brumbies ... are pretty good. They've got a strong midfield combination with Pat McCabe and also [Tevita] Kuridrani, they're two key players we have to try to shut down."
After losing their three away matches this season - against the Highlanders, Bulls and Lions - this match will indicate how far the Blues have come since their South Africa trip. The more controlled, clinical nature of their win over the Highlanders has raised hopes but they could be dashed by a poor performance in Canberra.
"If you want to be a contender you need to win away from home at some stage and keep your home turf pretty sacred," Kirwan said.
Blues team to face the Brumbies in Canberra on Friday night
15. Charles Piutau
14. Frank Halai
13. George Moala
12. Ma'a Nonu
11. Tevita Li
10. Simon Hickey
9. Bryn Hall
8. Steven Luatua
7. Brendon O'Connor
6. Jerome Kaino (c)
5. Hayden Triggs
4. Liaki Moli
3. Charlie Faumuina
2. Tom McCartney
1. Tony Woodcock
Reserves
16. James Parsons
17. Sam Prattley
18. Angus Ta'avao
19. Jordan Manihera
20. Luke Braid
21. Sonatane Takulua
22. Francis Saili
23. Benji Marshall