The Raiders enforcer hurt his AC joint taking a hit-up and was unable to finish the loss to the Sydney Roosters at Canberra Stadium on Saturday night.

Canberra coach Ricky Stuart was concerned after the game, with the club saying they would know more when he gets scans on Monday.

Papalii's a massive cog in the Green Machine's engine and they struggled without him in the second half.

It potentially sours the good news the Raiders were closing in on extending Papalii's contract for another two years to take him through to the end of 2024 - with the deal sealed potentially as early as this week when his manager comes to Canberra.

Brien Seeney, aka the NRL Physio, said the evidence pointed towards it being a grade two sprain - which normally took two-to-four weeks to recover from.

That would see him be anywhere between being back during the regular season or not playing again in the NRL in 2020.

It's the same injury future Immortal Cameron Smith missed four weeks with.

"Josh Papalii off with an AC joint injury. Usually considered a pain management/function issue, quicker return accompanied by painkillers/padding. But concern there for at least grade 2 sprain with him unable to return," Seeney tweeted.

The Raiders middle forwards have been hit hard with injury this season.

Sia Soliola (face), Emre Guler (ankle) and Corey Hursburgh (foot) were out injured, although Soliola could return for their round 19 clash against the New Zealand Warriors after Stuart ruled him out of this week's St George Illawarra game.

Canberra prop Ryan Sutton said while Papalii would be a massive loss he was confident they could cover Papalii's absence, pointing to Kai O'Donnell as someone who could come in.

"That's the good thing about this club we've got good middles that can do a job," Sutton said.

"I don't know what's happened with 'Paps', whether it's serious.

"If he is out for a few games or whatever it is we've got the players that can come in and do a job.

"We've got experience. I don't know if Sia's coming back, but Kai can do a job in the middle."

MORE RAIDERS NEWS

Papalii suffered the injury taking a hit-up in the first half.

While he was able to play on, he didn't return after being substituted in the 31st minute - having already amassed 112 run metres.

"Concerned," Stuart said when asked about Papalii's shoulder.

"We'll find out how serious it is. For Josh not to come back on, he's obviously feeling it.

"Joe Tapine is playing in the best form of his career too. If Josh is out we'll find someone to fill the jumper.

"I said it here seven or eight weeks ago when everyone wrote us off that you don't understand what's inside, no one gave us the ability to show you what's within the playing group, what the mentality is.

"Since the Josh Hodgson injury, Corey Horsburgh, Emre and the boys who are out in the middle, I think they've shown you how much spirit's in the joint."

NRL ROUND 18

Saturday: Canberra Raiders v St George Illawarra Dragons at Wollongong, 3pm.