Roosters assistant Craig Fitzgibbon has knocked back Newcastle's approach to replace Nathan Brown while the NRL has also put a line through any chance of Shane Flanagan returning to the game via the vacant Knights role.

The Knights are on the lookout for a new coach after mutually agreeing to part ways with Brown, with Fitzgibbon touted as one of the leading contenders to replace him and Flanagan the only available coach with an NRL premiership to his name.

Despite already being sounded out by Newcastle officials, Fitzgibbon has recently recommitted to the Roosters and given his word he will remain with the premiers.

"He's extended his time at the Roosters and he's happy there so he doesn't want to look into it," Fitzgibbon's agent David Riolo told NRL.com.

"He's only just done an extension and he's given his word that he won't go elsewhere.

"We've been approached by the Knights, which is flattering. I can see why they've approached him because he's probably the best coach not head coaching at the moment.

"That makes sense, but no he's already committed to the Roosters and he'll be staying there."

Fitzgibbon is the premiers' defence coach and is happy to continue his coaching education under Trent Robinson at this stage.

Flanagan has the credentials to turn the Knights into genuine contenders, but the NRL won't register his contract after he was banned indefinitely for breaching the terms of his previous suspension around the ASADA scandal of 2013.

Brown won't return to England to take on the job at St Helens vacated by Justin Holbrook's forthcoming departure to the Gold Coast Titans.

It's understood St Helens have already identified their coach and will announce it after this weekend's Challenge Cup final at Wembley.

While Fitzgibbon is already out of the running, fellow Roosters assistant Adam O'Brien remains as a leading contender for what would be his first head coaching role.

Former Panthers coach Anthony Griffin is also expected to come into the equation, so too Queensland coach Kevin Walters and former Canberra coach David Furner.

Like O'Brien, Melbourne Storm assistant Jason Ryles has also served an apprenticeship under Craig Bellamy and will be one of the names discussed as potential replacements for Brown.

Ryles was coincidentally "really, really close" to joining Newcastle's staff last year as an assistant to Brown, telling NRL.com that O'Brien's exit from Melbourne and his chance to rise up the Storm's pecking order convinced him to stay with the Victorian club.

Another former Dragons player, Trent Barrett, has landed a job as an assistant coach to Ivan Cleary at Penrith in 2020 but may also be an option.