Star halfback Will Genia believes the Rebels' two games in South Africa will make or break their Super Rugby season, but he will have to watch from Melbourne after injuring his hamstring.
Scans on Sunday revealed a grade one hamstring strain suffered in the three-point loss to the Jaguares on Saturday.
The defeat at AAMI Park forced the Rebels to surrender top spot in the Australian conference to the NSW Waratahs.
Genia grabbed at his hamstring and immediately left the field after his break set up a try for fullback Dane Haylett-Petty in the 49th minute.
"I haven't done a hammy before so it's all new to me," the Test No.9 said.
"We scored the try and I was trying to celebrate. I went to fist-pump but I felt something in my hamstring, like a deep cork."
Taking on the Bulls in Pretoria followed by the Stormers in Cape Town, the Rebels have never won in South Africa and Genia said a breakthrough victory would be crucial to their finals chances.
"I feel like this tour kind of makes or breaks our season as we are at mid-season and you don't want to start falling away," Genia said.
"Just being in that top six or eight teams, once you fall from that you are just chasing your tail the rest of the season."
Genia backed his replacement, Michael Ruru, to help halt the Rebels' slide down the Super Rugby ladder, while Australian under-20s halfback Harrison Goddard will also join the tour.
"Micky is a very good player and we see the game pretty much exactly the same," Genia said.
"He is a very similar player to me and he is younger, stronger, fitter and faster."
The 30-year-old was bitterly disappointed the Rebels blew a 19-6 lead to fall to the Jaguares.
"Without sounding arrogant or disrespectful, we should never lose that game, never," Genia said.
"The best thing about the match was that it's not skill execution or skill errors, it's just effort.
"You can flick a switch and fix that so I'm hoping the boys respond."