McCullough confirmed the news via social media, informing Broncos fans that he will require surgery.
McCullough suffered the injury when he was struck by teammate Adam Blair trying to make a tackle during the Broncos match against the Parramatta Eels.
McCullough will undergo a procedure to repair his anterior cruciate ligament (ACL) and medial collateral ligament (MCL).
The rehabilitation time for such an injury will give him some chance of being ready to return around the start of the 2018 NRL season.
“The general recovery for this injury is seven to eight months so he could be in danger of missing the first few weeks of next season,” said Broncos’ High Performance Manager Hickmans.
Brisbane will name it’s Round 22 side to take on the Gold Coast Titans on Tuesday.