And off-contract veteran five-eighths Keiran Foran and Benji Marshall are also not on O'Brien's shopping list despite the fact the Knights may look to strengthen their halves stocks if the right player becomes available.

Mullen's bid to return to the NRL after four years out for testing positive to a banned steroid back in November 2016 is considered a long-shot and would be one of the game's great redemption stories if he manages to pull it off.

News of him wanting to resurrect his playing career at the age of 33 after he completes 300 hours of community service following a cocaine supply conviction has so far been met with little or no response from clubs.

Knights CEO Phil Gardner told the Newcastle Herald on Tuesday Mullen's former club would not turn its back on the former captain once his ban is lifted. But O'Brien's lack of interest means if he was to return to the Knights, it would not be in a playing role.

There are suggestions Mullen, who now lives with his young family in Wollongong, may not even consider a move back to the region.

Asked about the fallen star's comeback plans during the build-up to Friday night's vital clash against the Sharks, the Knights coach admitted: "Coming back to the Knights is going to be a tall ask.

"The squads are reducing and there is going to be a whole lot of players that are currently in NRL squads that are going to be searching for a team next year.

"That in itself is a huge hurdle. That's not to say that he can't come back through some competition, whether it be NSW Cup or Queensland Cup or if it's Super League [in England]. That's up for Jarrod to descuss.

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"But I've got some young guys here in the halves that I'm trying to back in and that's also something against Jarrod coming to the Knights."

O'Brien met with CEO Gardner, footy general manager Danny Buderus and head of recruitment Alex McKinnon on Thursday to discuss the latest developments around the salary cap and roster.

They currently have only limited spots remaining but at this stage, don't know what the salary cap figure will be or the actual size of the playing squad, which could be reduced to 28 for 2021.

It's expected by the time Mullen is cleared to resume his career when his ban is up on January 16, the Knights playing roster will be all but finalised.

Mullen was rubbed out of the game for four years by the NRL after testing positive to the steroid Drostanolone back in November 2016. At the time, he was treating a chronic hamstring injury that had plagued the back-end of his career.

He has always maintained he took the steroid unintentionally after being given an injection by an unknown medico to help treat a chronic hamstring injury. His contract with the Knights was torn up, costing him $1 million.

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