Luki suffered the injury at training on Saturday morning and will undergo surgery in the coming weeks.
It's a bitter blow for the Queensland State of Origin prospect, who has endured a horror run of injuries since making his NRL debut in 2021.
Luki tore the same ACL in 2022, suffered a hamstring injury just after his return in 2023 and missed seven weeks of the last campaign with an ankle issue.
The 23-year-old is still yet to play more than 20 games in a season, but had endured a productive end to the 2024 season as the Cowboys made it to the second week of the finals.
Cowboys teammates celebrate with Heilum Luki (2-L) after a try against Canberra in round 15. (Lukas Coch/AAP PHOTOS)
The latest injury will sting for a North Queensland side that lost boom second-rower Kulikefu Finefeuiaki to the Dolphins for 2025, but could open up an opportunity for John Bateman to become a regular first-grader at his new club.
Wests Tigers released the Englishman from the remainder of his contract earlier this week to allow him to sign with the Cowboys for the next two seasons.
Fellow recruit Karl Lawton is another option to start on an edge alongside Maroons representative Jeremiah Nanai.
Luki's injury comes only one day after the Cowboys confirmed back Tom Chester was expected to miss the entire season with an ACL injury of his own.
Inspirational captain Jason Taumalolo is also sidelined from pre-season training after undergoing surgery on a Lisfranc injury, with his return-to-play timeline to be confirmed in the new year.
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