Johnston's third try of the 24-10 win at Stadium Australia was the seventh hat-trick of his NRL career and his second in as many games after scoring three tries against Parramatta in round 12.

Ironically, the speedster scored his first hat-trick against the Knights back in 2014 and he dished out the pain again on Saturday night.

It was salt in the wound for the Knights with strike centre Best taken off in the second half with a syndesmosis injury.

The 19-year-old has battled a number of injuries in his short NRL career and it comes just a fortnight after the Knights lost second-rower Tyson Frizell to a similar injury.

Even without Best, Saturday's effort was an improvement on last week's 40-4 flogging by the Eels.

Newcastle's cavalry is set to return in the coming weeks with David Klemmer due back from suspension against the Warriors next week and Kalyn Ponga, Mitchell Pearce and Hymel Hunt also in the frame to play.

"There's some help on its way but it is disappointing to lose potentially Bradman. We'll have to scan him and it's not looking good at the moment and it's heartbreaking, but we've just got to keep picking ourselves up and getting on with it," coach Adam O'Brien said.

"There's some short-term pain there but some young guys like Dom Young, an 18-year-old kid coming out from England, and he's just played his second NRL game.

"Matt Croker isn't even in our top 30, he works for a living and plays NSW Cup.

"They're all getting experience that will help our footy club long term."

Three days after NSW's big win in the Origin opener in Townsville, representative stars Latrell Mitchell, Jaydn Su'a and Daniel Saifiti all backed up for their respective sides on Saturday night, while Souths duo Damien Cook and Cameron Murray were rested.

After a brilliant performance for the Blues in their 50-6 win over Queensland, Mitchell had a mixed night for Souths.

The fullback was peppered under the high ball and bungled two catches but played out the whole game despite looking exhausted towards the end of the night.

"I was pleased with him. It was a great effort by him. He would have probably scored two tries if he wasn't as tired as he physically was," said Souths coach Wayne Bennett, who added he wasn't inclined to rest Mitchell.

"He gives himself enough spells, he doesn't need me.

"He's worked that part of the game out."

Sua played 35 minutes for the Maroons on Wednesday night and backed up brilliantly with a try to get Souths to a 14-point lead with 20 minutes to play.

The Knights had forced an arm wrestle until that point with Young and Kurt Mann getting Newcastle within two points at the break.

Australian Associated Press