After making his debut in 2014 and notching 129 games, Johnston wanted to be a one-club player.

But he faced insurmountable opposition to play fullback at the Rabbitohs with the arrival this season of superstar Latrell Mitchell.

Cook backed Johnston, who was part of Souths' 2014 premiership triumph, to find a new home and win a starting role at No.1 or continue on the wing.

He likened Johnston's situation to that of Adam Douehi, who has made an impact at Wests Tigers after being offloaded by the Bunnies at the end of last year.

"If Alex wants to be a fullback then he has just got to put his mind to it and work hard," Cook said on Tuesday.

"Like we've seen with Adam, he has gone over to the Tigers and is playing well there."

"He probably didn't think he was a fullback until we played him there last year, and now he is doing a great job for the Tigers.

"There is no doubt that AJ can do that as well."

Cook said that 25-year-old Johnston would do all he could to help the Rabbitohs claim another title, which would also help him secure a contract elsewhere.

"From speaking to him, he wants to play some good footy and help us win the premiership this year," Cook said.

"That will give him the best chance to get another contract ... it is a tough sport we play in sometimes, and that is one of the things that comes with it.'

Australian Associated Press