Newcastle have no guarantee of Jackson Hastings returning if they make the preliminary final, with coach Adam O'Brien describing the halfback's ankle injury as "significant".
The setback comes as the Knights confirmed middle forward Daniel Saifiti has been ruled out of Saturday's semi-final against the Warriors.
Former NSW representative Saifiti suffered a hamstring injury in last week's elimination final defeat of Canberra but had been expected to line up in Auckland.
He will be replaced in O'Brien's 17 by Brodie Jones, who has been in and out of the side this season.
The omission is a big one; Saifiti has averaged more run metres than any other Knights forward this year and has played in eight matches of the side's ongoing 10-game winning streak.
The Knights' organising half, Hastings suffered a low-grade syndesmosis injury in a hip-drop tackle a month ago and aggravated the issue on his return to the field in last week's NRL elimination final win.
The 27-year-old watched from the sidelines as Newcastle edged Canberra in extra time and was omitted from the extended squad for Saturday's semi-final against the Warriors.
Adam Clune will replace him in the side that takes the field in Auckland, having filled in for the last three games of the regular season.
Hastings' omission meant he was unable to make a late switch into the side if he returned to fitness but O'Brien said a miracle recovery was never on the cards.
"It was quite significant," O'Brien said of Hastings' injury.
"It hampered his ability to do his job, even in the early parts (of the Raiders game) but then he copped another knock to it.
"That'll lead to a more significant type of injury to what he had previously. He wasn't going to be able to perform his role so we just moved on."
The Knights may not have Hastings back for the preliminary final against Brisbane if they are able to pull off an ambush at Mount Smart Stadium, with O'Brien saying the injury could potentially sideline the player for longer.
"There could've been a little bit more damage to it," O'Brien said.
"But I'm not 100 per cent across it, I've just been bunkered down with this team and the guys we've got over here."
Newcastle will draw strength from winning all three of their games without Hastings to finish the regular season.
Clune filled in seamlessly as the Knights sealed their finals spot by easily accounting for fellow hopefuls South Sydney and Cronulla.
"I'm not going to downplay it," O'Brien said of Hastings' absence.
"He's a quality player, Jackson, I won't understate that.
"But what I will talk about is what Adam Clune's done for us in some pretty big games at the back end of this season.
"There were games that felt like semi-finals and Adam came in and did a great job."