Newcastle announced on Friday that they had signed Hetherington, the Canterbury firebrand, and Canberra workhorse Elliott for the next three seasons.

O'Brien said the pair exemplified the type of uncompromising qualities that made the likes of Danny Buderus and Paul Harragon club legends, back in the Knights' glory days.

"Both those guys compete," O'Brien said.

"They're fit, tough competitors, No.1

"That's what we need to fill the joint. The DNA of the Knights, back 20-odd years ago, the likes of 'Bedsy' [Buderus] and 'Chief' [Harragon] were tough, fit competitors.

"I think both of those guys represent that.

"Jack's got good leg speed for a big guy.

"The modern game requires players to have that leg speed ... Adam's in the same mould.

"I think both guys will compete in the game. They're tough, they'll speak at meetings.

"They're men."

Hetherington comes with a dubious judiciary record.

Since debuting for Penrith in 2018, he has played in 43 NRL games and been charged eight times with an array of illegalities, leading to a combined tally of 15 weeks on the sidelines through suspension.

O'Brien said that wasn't a deterrent. Nor does it appear to have worried the Raiders, Wests Tigers, St George Illawarra and Dolphins, who were also chasing the 26-year-old with lucrative offers.

"I'm not scared of that," O'Brien said.

"I think Jack understands. He's identified some tackle-technique issues that he's worked really hard at fixing.

"I think it's more about understanding the player and the timing of when he's got himself into trouble.

"Some of it could be around fatigue factor and when to get the interchange right around him."

Meanwhile, while the scoreboard suggested the Knights were blown away by Penrith in Sunday's 42-6 defeat at McDonald Jones Stadium, stand-in Panthers coach Cameron Ciraldo said it was a tougher contest than it appeared.

"I thought they played tough today," Ciraldo said. "I thought they showed a real set-for-set mentality and that tough attitude that Newcastle have."