The former head of the Australian Sports Anti-Doping Authority has thrown a spanner in the works of Jarryd Hayne's plans to play Olympic rugby sevens for Fiji, saying the departing NFL convert may not even be eligible for Rio.

Hayne made the stunning decision to walk away from the NFL and the San Francisco 49ers, saying he 'could not pass up' the chance to feature at the Olympic Games.

"The Olympics has been something I have admired since I was a little boy, and it is an opportunity I feel very similar to me joining the NFL," Hayne said on Monday morning.

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The 28-year-old was born in Sydney but his father, former NRL player Manoa Thompson, is Fijian

But former ASADA chief Richard Ings said Hayne was unlikely to play any professional sport, including rugby sevens, due to his time with the non-WADA compliant NFL.

"If Jarryd Hayne had been playing in WADA compliant sport he would have grounds to reduce the 6 months. But NFL is PED badlands. No chance," Ings tweeted.

"The 6 month return to competition rule is standard across most sports for athletes returning to international competition. And necessary."

Ings also tweeted at the official account of the World Rugby Sevens series, calling on the organisation to veto Hayne's move given he had not undergone the requisite six months of RTP (registered testing pool) testing.

Being barred from joining Fiji's sevens team would be an incredible shock for Hayne, who played eight games (one start) for the 49ers, registering 17 carries for 52 yards and six catches for 27 yards, while also returning eight punts for 76 yards.

He joined the Niners as an undrafted free agent in March last year, making the cut under former coach Jim Tomsula to complete his dream transition from rugby league to gridiron.

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Hayne has no plans after Olympics, says manager

Jarryd Hayne's manager says there is no Plan B post-Olympics for the former NRL star. ( AFP: Gregory Shamus/Getty Images )

Meanwhile, Jarryd Hayne's manager says the code-hopping star has not considered what his next move is post-Olympics after quitting the NFL.

The last six months has seen Hayne linked with an NRL return to the Sydney Roosters, but both Hayne and Parramatta have claimed he had signed a lifetime deal with the Eels when he made his 2014 exit from the NRL.

Hayne's manager Wayne Beavis says he has not considered his options come August, or even if his Olympic dream is over prior to that at selection.

"This is just a fantastic opportunity we want to grab and then we'll sit down and dust ourselves off after that and consider the next move," Beavis told Fox Sports.

"Plan B has never been discussed whether its rugby union, marbles, soccer or whatever. This came out of left field and we're going to run with it."

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Hayne himself again laughed off the speculation linking him to the Roosters on Twitter on Monday morning.

Responding to claims on the Nine Network that he would sign with the Tri-Colours in 2017, Hayne tweeted: "You an (sic) your mates been thinking that for 14 months".

When prompted further on whether the reports were correct, Hayne replied: "Why don't you ask your mate @BuzzRothfield said I would do it 6 months ago."

The NRL-turned-NFL-turned-Olympic hopeful then joked that a stint in the A-League could be his next option given his current run.

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When asked by a journalist whether he would then head to another sport or league such as the English Premier League after the Olympics, Hayne tagged the Western Sydney Wanderers and asked if they could do with a six-foot-two striker.

He told Triple M in February that if he ever made a return to the NRL, he would want it to be with the Eels.

"Like I always said, if I ever came back I'd go to Parra if they wanted me," he said.

"That was always something I said from the start. It's funny when you see media outlets say I'm doing this and I'm doing that."

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ABC/AAP