The Waratahs will be without star backs Alofa Alofa and Peter Betham © Getty Images

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Injuries to wingers Peter Betham and Alofa Alofa have forced NSW Waratahs coach Michael Cheika to call up a replacement for Saturday's Super Rugby conference derby with the in-form Western Force. Utility back Ben Volavola will fly to Perth as emergency injury cover after the Waratahs lost two outside backs in their 22-11 win over the Stormers on Saturday.

The win came at a cost, with Betham and Alofa both injured in the battle. Scans in Cape Town on Sunday confirmed a break in Betham's left foot, while fellow speedster Alofa suffered a medial ligament strain in his right knee. Betham and Alofa stayed overnight in Johannesburg before flying to Sydney on Monday rather than joining the team, who flew straight to Perth for Saturday's match with the Force.

Alofa and Betham aren't the Waratahs' only concerns, with second-rower Kane Douglas being monitored for a sore neck after a standout performance against the Stormers. Utility back Matt Carraro is receiving treatment for a cut to his head and Sekope Kepu picked up an ankle niggle but all three are still hopeful of playing against the fifth-placed Force on Saturday.

On the positive side, Cheika is confident No.8 Wycliff Palu will be fit to play after resting the ankle he hurt in Durban against the Sharks the previous week. Israel Folau should finally return to the starting side after a throat injury confined the star fullback to the sidelines during his week-long visit to South Africa.

The squad was due to arrive in Perth on Monday afternoon, leaving them just four days to shake off jet lag and prepare to face the team one rung below them on the table. NSW's win over the Stormers in Cape Town was just the second time in 20 attempts that a visiting side had won a game after crossing the Indian Ocean this season.

It lifted the Waratahs to fourth on the overall ladder with a game in hand, and second behind the Brumbies in the Australian conference. The Western Force are fifth overall after recording a club-record four-game winning streak with a 32-29 victory over Queensland on Saturday night at Suncorp Stadium.