With their hopes of making the top four dented by a 23-22 loss to the Sydney Roosters on Friday night, the Knights were also forced to endure a nervous wait on the results of two of their marquee players' scans.
Halfback Brown went down early in the second half, leaving coach Justin Holbrook immediately worried, and it was found that he had ruptured his anterior cruciate ligament.
The left knee injury means the Knights' $13 million signing will undergo surgery after being instrumental in their top-eight push this season.
It will also mean his hopes of playing for New Zealand at the Rugby League World Cup beginning in October are over, after the 26-year-old starred for the Kiwis in the Pacific Championships final last year.
Lucas, who made his debut for NSW in Origin II, had his leg trapped in a tackle with Siua Wong in the first half at McDonald Jones Stadium.
Scans revealed the second-rower had dislocated and fractured his left ankle and his timeline for return will be determined following surgery.
"Knights staff and teammates wish both players all the very best and look forward to supporting their return to play in the 2027 NRL season," a club statement said.
Newcastle have a tough few weeks ahead of them if they are to punch their ticket to the post-season, starting with premiers Brisbane at Suncorp Stadium next Saturday.
Melbourne hooker Harry Grant, meantime, faced a nervous wait after he picked up a potential season-ending hamstring injury in the Storm's 28-26 loss to South Sydney later on Friday night.
Scans revealed he has escaped with only a low-grade strain, and Melbourne expect him to be available following their round-24 bye.
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