The Newcastle Knights' mission to win a semi-final against the New Zealand Warriors on Saturday in Auckland has become a whole lot tougher, with key forward Daniel Saifiti ruled out with a hamstring injury.

The Knights confirmed the news in a short update on their website on Friday afternoon with teams trimmed to 19 at the same time.

Saifit's absence will hit the Knights hard, with the club's forwards performing strongly in recent times, but up against it in Auckland with the Warriors' pack led by Addin Fonua-Blake who is one of the key contenders to take a spot in the middle when the Dally M team of the year is named ahead of the grand final.

Hamstring injuries are notoriously difficult to manage as well, with few players able to come back from even a minor injury in a fortnight, meaning Saifiti, who suffered the injury at training, could miss a potential preliminary final if the Knights manage to beat the Warriors on Saturday.

Saifiti has been replaced in the side by Brodie Jones who joins the bench. The former Origin prop had only been named on the pine by coach Adam O'Brien on Tuesday.

Jones becoming his replacement also means the Knights go into the clash with the Warriors - which comes on a six-day turnaround after their epic 90-minute extra-time win over the Canberra Raiders last weekend - also means the club goes into the match with less size than they would have anticipated.

Kick-off is set for 4:05pm (AEST) - 6:05pm local time - on Saturday afternoon at a sold-out Go Media Stadium in Auckland.