Brett and Ripia sounded out
Crusaders and All Black stalwart Dan Carter will miss Saturday's Super Rugby clash with the Western Force in Perth due to a hamstring injury.
Carter skipped the Crusaders' two-match tour of South Africa to witness the birth of his first child.
The 30-year-old was on track to return against the Force, but had to shelve those plans after his training mishap on Tuesday.
Carter's injury means Tyler Bleyendaal will retain the No.10 jersey, despite his poor goal-kicking form in last week's 17-21 loss to the Sharks in Durban.
The Force will also enter the match missing a key player, with in-form lock Hugh McMeniman ruled out for at least six weeks after suffering a fracture to his left shoulder.
The Crusaders are eighth on the ladder with a 3-3 record, while the Force have just one win to their name.
Tom Taylor has already been scratched from the Force match and has returned to Christchurch.
Taylor, who has not played since the Crusaders' match against the Hurricanes on March 8, tweaked his medial knee ligament ahead of last week's match against the Sharks in Durban.
Although Blackadder has not selected a specialist flyhalf on the bench in the last two weeks - scrumhalf Andy Ellis was the back-up option - he will be anxious about the possibility of Bleyendaal breaking down so close to kick-off.
Before the Crusaders left for their two-match tour of South Africa last month it is understood they had contacted former Force and Hurricanes flyhalf Willie Ripia about being on stand-by.
They had earlier also sounded-out former -Crusader Stephen Brett, but he was unavailable.
Ripia has not played Super Rugby since being fired by the Force early last year after admitting he stole from teammates to fuel a chronic gambling addiction.
Like Brett, Ripia has recently completed a season playing in Japan.
Meanwhile, hooker Corey Flynn will be fit to play after he withdrew from the Sharks match because of a scratched eye.
Codie Taylor, who was rushed to South Africa to back-up Ben Funnell, has been sent back to Christchurch.
Sources: NZ Newswire & Fairfax NZ News