Mann-Rea (back) and Valetini (knee) have been left out of the Vikings side which will be chasing a fourth consecutive win when it plays the Melbourne Rising at Viking Park on Saturday night.

It's a blow for the ACT Brumbies teammates, who had been hoping to use the National Rugby Championship as a chance to get game time after their Super Rugby campaigns were cut short.

Mann-Rea and Valetini played just three games each in Super Rugby this year after major hamstring and knee problems. Now they're set for another stint on the sideline, although the Vikings hope their absence is only temporary.

Mann-Rea is battling to overcome a jarred back, while mystery still surrounds Valetini's third knee injury in seven months. It is hoped Valetini will get clarity by the end of the week, but it is not expected to be serious.

"I just found out Josh Mann-Rea is out injured," Vikings coach Nick Scrivener said on Thursday morning.

"He's stirred his back up a bit at training the other day. We thought he was going to be OK, but we made the call that he won't be right this week.

"Initially we thought [Valetini's knee] was going to be a bit worse than what it was. He's just getting some other opinions at the moment, so the call was made to rest him this week to know exactly what's going on.

"He hasn't properly been diagnosed with what it is yet, but the information I've got is that it's not all that serious."

Mann-Rea and Valetini's absence have opened the door for more sweeping changes to the Vikings side.

Connal McInerney has been recalled to the starting hooker spot and Vunipola Fifita will join him in the front row for his first run-on chance of the season.

BJ Edwards fills the hole left by Valetini at blindside flanker and Wests junior Brodie Leber, who was on the Australian sevens radar, gets his opportunity in the No. 7 jersey.

Jordan Jackson-Hope was unavailable for selection, giving Irae Simone a shot at inside centre and Len Ikitau a chance to return to the outside centre role.

Brumbies winger Chance Peni will play just his second game of the season as he looks to finish his year on a high after an injury and suspension plagued seven months.

Peni played seven Super Rugby games, but two suspensions cut short his season. In his comeback game in the NRC, he suffered a head knock and hasn't played since.

"[Peni] is another one who's been in and out. He's been training really well and frustrated that he's been out because of his head knock," Scrivener said.

"We take those things really seriously. But he's really, really keen to get back and play well for us. [Brumbies] contracted guys are looking forward as well and want to play well."

NRC ROUND 5

Saturday: Canberra Vikings v Melbourne Rising at Viking Park, 7pm. Entry is free.

Vikings: 1. Vunipola Fifita, 2. Connal McInerney, 3. Tom Ross, 4. Blake Enever, 5. Darcy Swain, 6. BJ Edwards, 7. Brodie Leber, 8. Ben Hyne, 9. Joe Powell, 10. Wharenui Hawera, 11. Chance Peni, 12. Irae Simone, 13. Len Ikitau, 14. Lausii Taliauli, 15. Andy Muirhead. Reserves: 16. TP Luteru, 17. Sione Taula, 18. Max Bode, 19. Liam Rasch, 20. Mitch Tye, 21. Ryan Lonergan, 22. Noah Lolesio, 23. Mack Hansen.