The NSW Waratahs have suffered a blow ahead of their Super 14 derby against the Brumbies tomorrow with former rugby league star Timana Tahu ruled out of the match.

Tahu has a hamstring strain and has been replaced on the left wing by 19-year-old Alfi Mafi.

Coach Ewen McKenzie preferred not to risk Tahu with NSW having a bye next week before heading to Christchurch to take on the Crusaders.

In better news for the Waratahs, fullback Lachie Turner will play after suffering an AC joint injury in last Saturday's win over the Highlanders in Dunedin.

His shadow, 18-year-old Rob Horne, will now make his Super 14 debut from the bench as Mafi's replacement.

NSW Waratahs team: Lachie Turner, Alfi Mafi, Ben Jacobs, Tom Carter, Lote Tuqiri, Kurtley Beale, Brett Sheehan, Wycliff Palu, Phil Waugh (capt), Rocky Elsom, Dan Vickerman, Will Caldwell, Matt Dunning, Adam Freier, Sekope Kepu. Res: Tatafu Polota-Nau, Al Baxter, Dean Mumm, Beau Robinson, Luke Burgess, Sam Harris, Rob Horne.

AAP