Mo'unga suffered the injury when making a tackle during his team's big 45-28 win over the Stormers in Christchurch last night.
Coach Scott Robertson will feel he is as well covered for Mo'unga's absence as he can be – Mitch Hunt performed extremely well last season when given the opportunity and veteran Mike Delany was signed in the off-season as a back-up for just this eventuality.
But, Mo'unga's quality in the first two rounds of the competition has been plain to see and the Crusaders will miss him. His time with the All Blacks last year has clearly taken him to a new level and he was playing with a poise which made him perhaps the best performing No10 in New Zealand at this early stage of the competition.
Robertson's men have done well to take 10 competition points from the first two matches but face a real challenge in the Hurricanes in Wellington next week already without the likes of Joe Moody, Owen Franks, Kieran Read and Israel Dagg.
But if any team can sail on regardless it is the defending champions. They play with a clarity, structure and hunger which means all in their squad are able to perform at a consistently high level, but the same can't be said of the Blues, who have lost halfback and skipper Augustine Pulu to an ankle injury, a key loss likely to hurt them badly.
Sam Nock, 21, will now be the starting No9 for the Blues on their tour of South Africa where they play the Stormers and Lions, two teams who would be tough enough for Tana Umaga's side as it was.
Pulu, who limped from the Eden Park pitch during his team's defeat to the Chiefs on Friday, is critical to the Blues. His toughness, especially on defence, gave his inconsistent side an edge but now Umaga must look elsewhere for that.