The NSW Waratahs have had to usher in teenage flyers Alfi Mafi and Robert Horne for Friday night's Super 14 blockbuster with the ACT Brumbies following the late scratching of Timana Tahu.

In a major blow for the Waratahs, Tahu was with forced to withdraw with a hamstring strain, prompting coach Ewen McKenzie to issue 19-year-old Mafi his maiden starting appearance in Super rugby on the left wing.

Horne will make his debut from the bench. The 18-year-old had been on standby to replace fullback Lachie Turner but in one welcome piece of bright news for the Waratahs, he wasn't needed to start after Turner proved he had overcome an AC joint injury sustained in last Saturday's win over the Highlanders in Dunedin.

Tahu's withdrawal - which comes two days after the dramatic revelation that Brumbies fullback Julian Huxley has a brain tumour - coupled with a mass exodus of experienced campaigners from both sides over the past two seasons, has the derby now being billed as "the battle of the baby backlines".

With no Tahu, no Chris Whitaker, no Mat Rogers, no Wendell Sailor and no Morgan Turinui, the average age of the new-look NSW backline is just 23.

The Brumbies, with no Huxley, no George Gregan, no Stephen Larkham, no Matt Giteau and sans injured stars Stirling Mortlock and Clyde Rathbone, average only 24 years a player across the backline.

"It's like a schoolboy reunion. Most of those guys I've played with, so it should be good," Mafi enthused.

"I'm pretty excited. I've been waiting for this for a long time, so I just can't wait to get out there and play.

"I'm just going to try to get out there and play like (Tahu) does and hopefully things will fall into place and I will take my opportunities."

Tahu said he first felt his hamstring tighten at training on Wednesday and a subsequent MRI scan revealed a small tear.

"I'm really disappointed to miss this game because obviously the local derby is one of the biggest matches," he said.

"But it's too big a risk to play this week."

Coach McKenzie said the fourth-placed Waratahs decided they were better off taking advantage of next week's bye and having Tahu available for the round five showdown with the unbeaten Crusaders in Christchurch.

"Obviously you don't want late changes and it's disappointing to lose Timana," McKenzie said.

"He'd love to be playing but the main thing is that we don't turn it into a bad injury. At the moment, it's not a bad injury. But it's not good enough to play tomorrow night.

"Alfi's been playing off the bench for us every week and he had a great pre-season. So it's a disruption, but one we're confident we'll get through."

Waratahs: 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.

Brumbies: Mark Gerrard, Peter Playford, Sanualio Afeaki, Tyrone Smith, Francis Fainifo, Christian Lealiifano, Patrick Phibbs, Stephen Hoiles, George Smith (capt), Mitchell Chapman, Mark Chisholm, Adam Wallace-Harrison, Salesi Ma'asu, Saia Faingaa, Nic Henderson. Res: Huia Edmonds, Ben Alexander, Peter Kimlin, Julian Salvi, Joshua Holmes, Asusipa Taumoepeau, Tim Wright.

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