Anti-doping laws could stop Jarryd Hayne from competing at the Rio Olympics, according to former ASADA boss Richard Ings.
Hayne quit the NFL on Monday morning to attempt to earn selection with Fiji in rugby sevens at the Games.
However Ings said World Rugby rules require players to be in a registered testing pool for six months to be eligible to compete at sanctioned competitions.
And according to the former ASADA chief, Hayne's stint in the NFL does not fit the requirements.
"If Jarryd Hayne had been playing in WADA compliant sport he would have grounds to reduce the 6 months," Ings tweeted on Monday morning.
"But NFL is PED badlands. No chance."
Ultimately, the decision on whether Hayne would be cleared to play will come down to the International Rugby Board, while the former NRL star could appeal any decision which goes against him.
But Ings believes the restriction is a vital one.
"The six-month return to competition rule is standard across most sports for athletes returning to international competition. And necessary," Ings tweeted
"The rule equally applies to reinstatement or to new players. Sensibly it must or any NFL player could bulk up and play."
Hayne's hopes of being cleared to play should increase though given fellow NFL-convert Nate Ebner debuted for the United States at last month's Singapore World Sevens tournament - just a month after he left the New England Patriots.
Ings, who was chief of ASADA for six years, also indicated similar rules apply to the NRL.
Hayne "retires" from NFL, turns focus to Sevens for Fiji.
The 28-year-old will travel to London tomorrow to join up with the Fiji Sevens side ahead of the World Series finale at Twickenham.
The former NRL star and San Francisco 49ers running-back will join Ben Ryan's side as a relative Sevens rookie and just three months out from August's Rio Games.
Having taken a step closer to a second successive World Series title with their performance at the penultimate round in Paris, coach Ryan is looking to further his options before naming his squad for the final round at Twickenham and also for the Olympic Games in August.
"I've had a few conversations with Jarryd and followed his career in the NRL in Australia, in the Bati with Fiji, and more recently with the NFL as a running-back for the 49ers," said Ryan.
"He got a taste for sevens when he was with the boys in Sydney and that sowed a few seeds, and that has just got bigger. He has had a chat with the 49ers and his agent, and I have with everybody else to make sure he is on our accreditation list and to ensure he ticks our boxes, he is a Fijian passport holder, all those sorts of things.
"I have no promises he is going to make the 12, but we will see how he goes. He will then come into Fijian camp for the Olympic period. It is a huge challenge for Jarryd but if he gets into the squad it is only going to be on form, because he is a blinding rugby player. If he doesn't make it, is just shows how good this Fijian sevens side is.
"It is a no lose situation for me. It is no lose for Jarryd either, he gets himself into the possibility of being a part of this journey, and the boys are all going to learn off him as well. He comes in with an A* but it is up to him to hang onto that, it's really exciting and really what the Olympics is all about."
Hayne made 176 appearances for the Parramatta Eels in the NRL before making the move to the NFL and has represented both Australia and Fiji at Rugby League World Cups in 2013 and 2008 respectively.
The running back has been released by San Francisco ahead of the London Sevens and the road to Rio.
Rugby sevens makes its debut in the Olympic Games this August in Rio and Fiji will be aiming to win a first-ever medal for their country.
A squad of 24 will assemble in June for Fiji's training camp before the coach name's a final 12.
"I think any coach would say they'd happily take my job. It is a good challenge to have," added Ryan. "I want to create this depth and I like to be challenged, you don't just want to keep skating along. I want someone to come in, who is going to see things differently, I am going to have to make Jarryd is up to speed and I haven't forgotten things.
"It's like the overseas boys in Paris. I am checking their technique is right at the breakdown, it keeps me checked as well so I like bringing new players in. It doesn't make my life any easier but for good reasons it is for us all to get better.
"The camp will be like Britain's Got Talent or the X Factor, boys are going to get chopped until we get down to 12, it will be like a shootout. All these stars are going to be back, everyone will be fighting hard.
"I have seen a glimpse of it this week, there is no space to play the competition is so high, it's always at 110%. We are going to have that for six or seven weeks. I need to get that balance right, but I will stay true to my values and pick on form."