Butler has stepped up to fill some of the biggest shoes in world rugby, given the task of replacing injured superstar David Pocock.

But unfazed by the challenge of wearing the famous Brumbies' No.7, Butler has established himself as one of the team's most consistent performers despite his low-key status.

The Brumbies' No.7 jersey carries an expectation of greatness. It started with inaugural captain Brett Robinson, was handed to Australian rugby legend George Smith, spent some time with Hooper before moving to Pocock.

"But we don't need JB to be Poey or George Smith, he does what we need him to do," prop Ben Alexander said.

"We just need him to do that role in our squad, Scott Fardy is there helping him out and he's been outstanding.

"He's not overplaying his hand trying to be the guys that he's replacing, he's doing what [director of Rugby] Laurie [Fisher] has asked of him and he's a huge part of the reason why we're [in the semi-final].

"He's massively under-rated, he's like [lock Sam] Carter. No one was talking about Carts last year, but he made his Wallabies debut this season."

The Brumbies recruited Hooper as a talented teenager with the view he would become the club's long-term openside flanker to replace Smith.

But just as Hooper's career started to take off, he defected back to Sydney to the Waratahs and will lead them into battle against the Brumbies at Allianz Stadium on Saturday night.

The Brumbies recruited Pocock – one of the world's best openside flankers – to replace Hooper, but the marquee man has suffered back-to-back season-ending knee injuries in the past two years.

"It's tremendously pleasing [to see Hooper playing well], it's taken his game to far greater levels than when he left us," Fisher said.

"That's one of the reasons we're in coaching. You want to win championships and you want to win games, but you want to develop people and players. To be involved with a guy who is now one of the world's best is one of the reasons you're in the game."

Former Australian under-20s back-rower Butler joined the Brumbies at the start of the year hoping to cement a spot on the bench.

But the 23-year-old has started in every game this year and has re-signed to stay in Canberra for the next two seasons.

"Maybe externally and maybe because he's unknown he seems under-rated, but from day one we knew he was skilful," said Brumbies coach Stephen Larkham.

"We haven't been surprised at all how he has gone."