Ponga is expected to miss months rather than weeks with the foot injury he suffered in the Knights' thrashing from Canterbury, their fifth loss from seven starts in 2024.
Armstrong, 23, joined the Knights' top-30 squad this season after two years in NSW Cup and has already attracted Super League interest, despite not yet debuting at first-grade level.
He gets his first chance in the top grade on Sunday against the Dolphins in Brisbane, where Tyson Gamble and Jackson Hastings remain the capricious Knights' halves pairing.
Dane Gagai will captain the Knights on return from his calf injury.
St George Illawarra winger Zac Lomax retains his spot in the centres despite Jack Bird recovering from concussion for the Anzac Day clash against the Sydney Roosters.Â
The irony is unlikely to be lost on Lomax, who has only been moved from wing to his preferred position after signing with Parramatta.
Struggling South Sydney have recalled State of Origin hooker Damien Cook for Thursday's clash against Melbourne. He is joined in the 17 by newly fit Jai Arrow and Tyrone Munro's injury replacement Jacob Gagai
Experienced second-rower Kurt Capewell has suffered a throat injury and misses the Warriors' Anzac Day clash against Gold Coast, replaced in the starting side by Jacob Laban.
Elsewhere, Ethan Sanders replaces Daejarn Asi (head knock) in the halves for struggling Parramatta, whose coach Brad Arthur has made a handful of other changes in response to the 44-16 loss to the Dolphins.
Sean Russell, Bryce Cartwright, Luca Moretti and Blaize Talagi all drop out of the 17 that lost in Darwin, where Arthur said his side "gave up" in the second half.
Maika Sivo returns from his spell in reserve grade to face Manly on Friday, with Brendan Hands, Makahesi Makatoa and Kelma Tuilagi all joining the Eels' bench.
Nathan Brown and Josh Aloiai bolster Manly's forward pack on return from a knee injury and suspension, respectively.
Brisbane welcome star trio Payne Haas, Adam Reynolds and Brendan Piakura back from injury for Saturday's clash against a Wests Tigers side that hands rugby union convert Solomon Alaimolo his NRL debut.
But five-eighth Ezra Mam (finger injury) and Selwyn Cobbo (shoulder) are out for the Broncos, replaced by Jock Madden and Deine Mariner, respectively.
Cronulla's Daniel Atkinson will line up in the halves against Canberra in place of Braydon Trindall, who has been stood down after failing road-side drug and alcohol tests on Monday.
The Raiders are fielding a new-look halves pairing themselves, with Kaeo Weekes looking at a long stint replacing the injured Jamal Fogarty (bicep) at halfback.
Nathan Cleary has been named to return from his hamstring injury as Penrith travel to Townsville to face out-of-form North Queensland.
Cowboys coach Todd Payten said he would contemplate swinging the axe after last week's loss to Cronulla, but only Jack Gosiewski has been dropped from the 17, replaced by Thomas Mikaele.
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