Mullen has asked the Australian Sports Anti-Doping Authority to test his B sample after an initial sample turned up a positive result for the prohibited drug drostanolone.

The National Rugby League suspended the 29-year-old provisionally on Tuesday, and he faces a ban of up to four years if his B sample comes back positive.

A stunned Gidley told a media conference on Tuesday afternoon that his former teammate had “gone outside the club and taken advice from someone, and now he's going to pay a price”.

Gidley said he had spoken to Mullen twice on Tuesday after the club was made aware of the positive test but the player had not offered an explanation.

“It wasn’t a long conversation,” he said. “It was just more of making him aware that we were aware. 

“Now there’s a process to follow and we need to respect the process, so it wasn’t appropriate to go into detail.

“He’s devastated, so it’s important we consider Jarrod’s wellbeing through this process. This is going to be incredibly difficult for him. 

“I just made it clear that while we are extremely disappointed in what’s transpired here, that we will continue to stand behind him and support him – from a wellbeing point of view, that is – through this process.”

The club boss would not say whether the NRL-owned Knights would continue to pay Mullen during his provisional suspension. The former Origin player is halfway through a four-year contract worth at least $700,000 a year. 

Gidley, who played with Mullen when he started in first grade in 2005 and 2006, said Knights players were well educated on the perils of doping.

”They are fully aware of the consequences,” he said. 

“If they go outside of the club and do things that we are not aware of, then there’s not much we can do. 

“But they are fully aware of the risks. There’s been too many examples in professional sport of what happens when they do go outside of the club and the people they trust and take advice from people that aren’t professionals, and they’re the consequences.”

Asked whether Mullen might have been desperate to recover from hamstring injuries last year, Gidley said: “I’m not sure. I didn’t go into detail with Jarrod as to why he went down this path.”

Mullen is the first NRL player to test positive to steroid use since former Roosters forward Martin Kennedy was banned for two years and nine months 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 said on his Twitter account.

Ings said talk of Mullen’s career being over were “a bit premature”.

“He has a positive A sample. Best to wait till we have much more information before assuming such outcomes,” he tweeted.

Gidley said coach Nathan Brown was “bitterly disappointed” with the turn of events. 

“I mean these players, we work closely with the players, they become like a family. We spend so much time together, so it’s never nice to see one of your players go down this path, so that’s why it was important we spent some time with Jarrod today.

“ . . . Again, the message for all players in professional sport is listen to the people inside your club and take advice from them.”

Mullen has played 211 games for the Knights and is one of their highest-paid players. He has been an automatic selection in the halves for 10 years but has been training at hooker in the pre-season.

Gidley said Mullen was under no more pressure to perform than other players at the club after two wooden-spoon seasons.

A long ban would likely end Mullen’s career, and Gidley agreed the outcome of a confirmed positive drug test would be “pretty severe”. 

“I don’t want to speculate around what the B sample might come back and tell us, but I think we are all aware of the consequences in sport if that’s the case.”

The Knights have a long history of drug problems, including former champion Andrew Johns’ admission that he took cocaine during much of his career, but Gidley viewed the Mullen scandal as an “isolated incident”.

“We had commanders and senior detectives from the police here last week talking to our playing group about separate issues and separate threats to the game.

“All we can continue to do is educate the players and keep reinforcing the consequences if they go outside the club and take advice from people who aren’t employed by the club who have their own best interests at heart.”

The Knights are due to send a squad to the Auckland Nines on February 4 and 5 and kick off the NRL season against the Warriors in New Zealand on March 5.

The Newcastle Herald tried unsuccessfully to contact Mullen.

His agent, Steve Gillis, declined to comment.