Holbrook is expected to be one of several options for Newcastle after O'Brien officially agreed to walk out on the final two years of his contract.
Current Knights assistant Blake Green is considered the front-runner for the job, but as many as four others have already been floated as potential alternatives.
Several challenges await whomever does take over, with Knights legend Andrew Johns pointing to issues in the club's junior pathways.
Johns claimed on a Nine Network podcast this week it would require a long-term fix, and resurrecting bottom-club Gold Coast would be "kindergarten stuff" compared to the job needed at the Knights.
Beyond junior pathways, the next coach must also settle Newcastle's halves, ensure Kalyn Ponga exit rumours remain as only speculation and work with Dylan Brown's lucrative 10-year deal.
Holbrook is contracted with the Roosters until the end of next year, having landed at the Tri-colours on a three-year deal after being axed by Gold Coast in 2023.
The Roosters are no stranger to losing assistants to top jobs with Craig Fitzgibbon, Jason Ryles, the late Paul Green and Jason Taylor all taking similar paths.
O'Brien was also Roosters assistant before taking on the Knights posting, reportedly causing issues when he left mid-contract for the Newcastle job.
But Robinson said he would fully support Holbrook applying to be the Knights' next coach.
"You always support your coaches in wanting to become head coaches," Robinson said.
"If they want to become head coaches, they're going to really strive to be the best assistant coaches possible.
"And Justin's a really good coach whose focus is obviously on the best for us.
"It (Newcastle) is also a club where he's played for and spent time as well, so I'm sure it'd be of interest to him.
"When they're up front and honest with that's the way to do it, (like when) Greeny, JT, Fitzy and Justin when he went to St Helens (in 2017)."
Like Holbrook, Brad Arthur could bring experience to the role with the former Parramatta coach currently with Leeds in the English Super League.
Willie Peters is another option after taking Hull KR to the top of the Super League ladder, while Newcastle's other assistant Brian McDermott has also been mentioned.
Green, however, remains the frontrunner, having gone from player to assistant at Newcastle after his retirement in 2021.
"I won't have any input (on a replacement), that is a club decision," O'Brien said Thursday.
"But Greeny has been unbelievable for me.
"I don't think there is a player in this club, male or female, from age 15 to 30, that he doesn't know.
"He has his fingerprints on the pathways, which is a really strong point for this club moving forward."
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