Fitzgibbon suffered a suspected torn medial ligament in his left knee late in the first half of Sunday's 26-4 victory against Wests Tigers at McDonald Jones Stadium.

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He was sent for scans on Monday and the early indications were that he would be missing for at least the next few games.

Knights coach Adam O'Brien said after Sunday's game he was "not too sure" about the severity of Fitzgibbon's injury, but the club's medical staff were confident he had not damaged his anterior cruciate ligament.

The 28-year-old back-rower has endured a challenging time in the past 18 months.

At the end of 2020, he underwent pelvic-mesh surgery in the off-season, only to then need a shoulder reconstruction months later that sidelined him until round nine.

He subsequently aggravated the latter injury last August and required another operation that curtailed his 2021 campaign after only 10 games.

After a strong pre-season, he looked set to make up for lost time this year, only to suffer an ankle injury in Newcastle's round-one boilover against Sydney Roosters, followed by the tackle that damaged his knee against the Tigers.

While Fitzgibbon will be missed, Newcastle have a host of reinforcements on the horizon. Knights captain Kalyn Ponga and front-rowers David Klemmer and Daniel Saifiti head the list of likely inclusions.

Ponga and Klemmer both withdrew from Sunday's game because of minor knee issues, while Saifiti looks set to resume after suffering a fractured tibia in Newcastle's opening trial match.

In addition, back-rower Brodie Jones is ready to return from a quadriceps injury that ruled him out of the trials, and experienced Sauaso Sue could be available for his first game since off-season shoulder surgery.

If all those players are available, O'Brien faces some tough selection decisions, given how well his fill-ins have played in racking up back-to-back wins that have propelled Newcastle to the top of the points table.

"The elephant in the room is the amount of guys who were missing," O'Brien said of Sunday's win.

"I think we had half a dozen there.

"We had seven players this week who didn't participate in one training run, not even the captain's run. They just turned up to play.

"But we've worked really hard on our culture, and that's as good a next-man-up mentality as I've seen from the boys, to defend the way they did."

O'Brien acknowledged, however, that the Panthers - who are also unbeaten this season - would be a step up in class.

"They're a classy footy team ... they know the way they want to play, they know their defensive system so well," O'Brien said. "It's a good test for us. We've started well but this is a big test."

Penrith will again be without champion halfback Nathan Cleary, who is yet to return from shoulder surgery.