The ex-South Sydney No.9 said in the pre-season he had been hopeful of a new deal with the Dragons, whom he joined on a two-year contract in 2025.
But in a sign of the last-placed joint venture's reconfiguration for the post-Shane Flanagan era, the club announced on Friday that Cook would not be renewed.
He has been linked with a move to English Super League side Castleford, who count former NRL players Blake Taaffe, Mikaele Ravalawa and Joe Stimson on their roster.
The news comes a day before the winless Dragons' round-10 clash with Newcastle in Wollongong, and is likely to clear the way for Jacob Liddle to start at hooker next season.
"My full focus right now is on doing everything I can for this team and this club over the remainder of 2026," Cook said in a press statement.
"I've loved my time at the Dragons, and I'm determined to finish it the right way."
Cook, who turns 35 next month, has provided solid leadership experience for St George Illawarra, but has not recaptured the dazzling best of his heyday at South Sydney.
In his pomp, Cook was the most dangerous ball-running hooker in the league, amassing 17 State of Origin appearances for NSW and winning Dally M Hooker of the Year in 2018.
After 207 games with the Rabbitohs, Cook rejoined his junior club last year and is set to play his 250th NRL game against Penrith at Magic Round.
The Knights are favourites to extend the Dragons' losing streak to 13 games this Saturday.
But coming off the bye, Saints have given Holbrook reason to be wary in their second game under interim coach Dean Young.
"We're probably getting to the most dangerous time (to play them), coming off a bye and a new coach so they've had two weeks to prepare," Holbrook said.
"They'll be feeling really good about themselves and it's up to us to nullify that and make sure we play well."
Knights co-captain Kalyn Ponga plays his 150th NRL game, almost 10 years after making his debut for North Queensland during the 2016 finals series.
Ponga has contended with significant injury setbacks en route to the milestone, most recently overcoming a hamstring issue ahead of last week's defeat of Souths.
In Holbrook's eyes, the recently re-signed fullback is a better player than ever.
"He's shown that in three games this year, which is great, it's great to have him back on the field," Holbrook said.
"Great achievement obviously and in just nine more games' time it'll be 150 (for the) Knights and that's even better. Really good accolade for him and a lot more to come."
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