Rebels scrum-half Luke Burgess has been ruled out for two months after injuring his knee in Saturday's win over the Reds in Brisbane.
The injury is a blow to the Rebels, who are already without the services of fellow scrum-half Nic Stirzaker, leaving rookie Ben Meehan and experienced injury replacement Josh Holmes to battle for the nine jersey against the Waratahs in Melbourne on Friday.
Rebels' Head of Athletic Performance Bryce Cavanagh was disappointed to lose Burgess, but said the prognosis was originally feared to be far worse.
"We thought we might have seen the end of Luke's rugby season, so a six-eight week lay-off whilst not ideal is certainly far better than what we originally feared," he told the Rebels' official website.
"Fortunately, Ben Meehan has seen plenty of Super Rugby action and from a performance point of view is match fit and ready to go after combining his five substitute appearances with some full games in the Dewar Shield.
"It's a similar story with Josh Holmes, who joined the squad in very good condition following a strong start to the Shute Shield season with Warringah."
Cavanagh was pleased that there weren't any further additions to the injury list following a desperate game against the Reds that was not short on physicality.
"Both teams were throwing everything into it, and we saw two Reds players and Luke leave the field early with injuries," he added.
"We're pleased that despite the immense physicality on display, no other Rebels picked up knocks.
"Especially pleasingly, Toby Smith was dominant on his return from a shoulder injury with no issues to report, and Pat Leafa and Tom Kingston were able to step up and play a great 80 minutes each."
Irish recruit Tom Sexton and Lachlan Mitchell continue their rehabilitation from season-ending knee injuries, while Male Sau and Shota Horie are unavailable for selection again this week following their departure for crucial Rugby World Cup qualifiers with Japan.
Australian U20s captain Sean McMahon is available before heading off to join with the national squad on Saturday morning.