A win will see them head to Brisbane the following week for a spot in the grand final.
The win over the Raiders came with Hastings off the park during the second half after he suffered a reoccurrence of his ankle injury that had kept him out of the closing weeks during the regular season as the Knights won ten games on the trot.
Asked if the injury could keep Hastings out for more than just this weekend's game, O'Brien admitted that was the case.
"Potentially. I'm not across everything just yet, but potentially. I think there could have been a little bit more damage to it," O'Brien admitted to the media in Auckland after the Knights' arrival ahead of Saturday's game.
Hastings has struggled with leg injuries ever since he was the victim of a horror hip drop tackle last season and admitted after reinjuring it last week that it would likely stick with him for the rest of his career.
So determined was the halfback to play that he told The Sydney Morning Herald after the win over the Raiders that he wasn't going to get scans, and would instead focus on rehab in an effort to run out with his teammates.
While he won't play in Auckland, O'Brien said that while he is a big loss, Adam Clune has been excellent and would hold up his end of the deal.
"He's a quality player Jackson. I won't understate that. I don't want to downplay it," O'Brien said.
"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."
Kick-off is scheduled for 4:05pm (AEST), 6:05pm (local) on Saturday evening in Auckland.