Western Force coach Richard Graham admits it's make or break time for his battling side if they are to stay in contention for a Super Rugby finals spot.

The Force, who have posted just one win and a draw from five outings, face a defining encounter against the Melbourne Rebels in Perth on Saturday night before embarking on tricky assignments against the Waratahs (home) and Brumbies (away).

The three derbies will go a long way to determining the final make-up of the Australian conference and Graham conceded the Force needed to make their move now if they were to stay in the hunt for one of six finals positions.

"It clearly is (a season-defining stretch). We are six games into the season (and) these next three games are going to be very important for us," Graham said on Thursday.

"It will set you up as to whether you go on and finish strongly in the conference or whether each week you've got to fight for survival.

"It either determines the outcome of our conference or whether or not you finish second.

"So this weekend is very important for us."

Former Hurricanes five-eighth Willie Ripia failed in his bid to convince Force fitness staff he had fully recovered from a foot injury, leaving James O'Connor to again don the No.10 jersey.

"Willie trained well with the team but I felt as though he isn't quite ready to play 80 minutes of Super Rugby. He isn't far away," Graham said.

Inside centre Gene Fairbanks returns to the side after missing last week's loss to the Stormers due to a virus, while hooker Nathan Charles replaces Ben Whittaker in the run-on side.

Graham said his players were eager to atone for the woeful 51-16 defeat to the Stormers in Cape Town, where the home side ran in six tries to one in a brutal display.

"I hope they're hurting because the performance was obviously not good enough," Graham said.

"We pride ourselves in the way that we prepare for games and our preparation throughout the week we felt was good but going into it, obviously we'd fooled ourselves."

The Force will be on their guard against the unpredictable Rebels, whose two wins have come the week after humiliating losses.

"Everyone thought they were going to be the whipping boys but they've won two games and been very competitive at home, so this is going to be a tough game for us," Graham said.

"They've got a strong forward pack, (Danny) Cipriani is unpredictable at 10 and Stirling (Mortlock) is carrying well for them, he's getting across the gain line so he's someone we're going to have to look after first phase."

Meanwhile, Geoff Stooke stepped down as RugbyWA chairman on Wednesday night after 25 years of service to rugby at board level in Western Australia, with Dr Russel Perry endorsed as his replacement.

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