Both are big men who possess finesse as well as power, and it's a potential combination that will interest not only Umaga but also All Blacks coach Steve Hansen.
In selecting his match-day squad, Umaga said he was repaying pre-season form. But his hand has also been forced by the injuries to Williams, hooker James Parsons and late returns from All Blacks Patrick Tuipulotu and Jerome Kaino.
Tuipulotu's return was complicated by a well-publicised positive and then negative drugs test, while Kaino's has been made more difficult by the sudden death recently of good friend Sione Lauaki.
Kaino was understood to be targeting a return in round two against the Chiefs in Hamilton, but a round three return against the Highlanders at Eden Park will also now be an option.
In the meantime the importance of a good start to the Blues can hardly be overstated. In his first season in charge Umaga ironed out of few of the inconsistencies which have plagued the Blues for years, and a good performance away from home, no matter that it's against one of the competition's lesser lights, will continue that trend.
"The way we're training and the things we're getting out of these guys, we're very happy with where we are at," Umaga said.
"The group is pretty committed to starting well and doing well, and know there's a lot of competition and a lot of people that are coming back who have some reputations; they want to put their best foot forward to stay where they are."
A major positive in the team's pre-season matches against the Hurricanes and Chiefs has been the scrum, and in particular the performance in the set piece by former Chiefs prop Pauliasi Manu. Consistency in that area will put the Blues on the right track as All Blacks prop Charlie Faumuina, named on the reserves bench, suggested today.
Faumuina said: "We really need to set the platform this year and that starts with us in the tight five really taking it to teams and winning that battle so we can give these guys some good ball."
Blues team to play the Rebels at AAMI Park, Melbourne, on Thursday, kick-off 9.40pm NZT is:
15. Michael Collins
14. Matt Duffie
13. Rieko Ioane
12. Piers Francis
11. Melani Nanai
10. Ihaia West
9. Augustine Pulu
8. Akira Ioane
7. Blake Gibson
6. Steven Luatua
5. Gerard Cowley-Tuioti
4. Jimmy Tupou (c)
3. Sione Mafileo
2. Matt Moulds
1. Pauliasi Manu
Reserves: Hame Faiva, Sam Prattley, Charlie Faumuina, Scott Scrafton, Murphy Taramai, Sam Nock, George Moala, Rene Ranger.