Jarrod Mullen's NRL career hangs in the balance after the Newcastle star was provisionally suspended by the NRL for testing positive to an anabolic steroid.
The Knights and NRL announced today that Mullen had returned a positive sample to Drostanolone in an out-of-season test in late November.
The test came just a week after the 29-year-old aggravated a recurring right hamstring injury. Under the NRL's drugs policy, Mullen is facing a maximum four-year ban but can decide to have his B-sample analysed by the Australian Sports Anti-Doping Authority.
"The club is obviously extremely disappointed," CEO Matt Gidley said in a statement.
"The club has a strict governance program, the players are regularly educated and fully aware of the consequences of going outside our governance guidelines.
"From here, there is a formal process we need to follow under the guidelines. We need to respect the process and in the interim the club will continue to monitor Jarrod's welfare."
A shocked Gidley later told reporters that there isn't any more education the game could provide to its players regarding the consequences of drug use.
"One of the things I'm really proud of in terms of our game is the way we educate our players now and the NRL has played a huge role in that," he said.
"We can't control what players do when they leave the club and who they take advice from.
"I can't believe it. It's hugely disappointing. For a player that I've known for 20 years. He's an experienced player. It's disappointing for everyone involved."
ASADA said it had completed the test on November 29.
"ASADA undertakes a comprehensive anti-doping program to ensure the integrity of the NRL competition," ASADA said in a statement.
Mullen is the first NRL player to test positive to steroid use since former Sydney Roosters forward Martin Kennedy was hit with a two years and nine month ban in 2015.
Former ASADA boss Richards Ings said Drostanolone, which is also known as Masteron and is primarily used for the treatment of breast cancer patients, was a very serious steroid.
"He'll need to show the source of the steroid and that he exercised extreme caution to avoid ingesting it," Ings tweeted.
Mullen has played 211 games for the Knights since making his debut in 2005 and was considered an integral member of their line-up ahead of the upcoming 2017 season.