He has raised more than £13 million for charities involved in supporting people with the disease and seeking a cure. His most recent challenge was seven ultra marathons in seven days last month.
"He is our Jason Robinson,"Sinfield said. He played against Robinson during the pair's days in rugby league.
The comparison framed a fitness call that kept Feyi-Waboso in Sunday's match despite the injury, with coaches weighing his wish to continue against the need to manage him.
Feyi-Waboso suffered the ankle injury in the 32nd minute of Sunday's 45-22 win over the Wallabies. He played on, hobbled his way to half-time and was then replaced.
Sinfield said Feyi-Waboso's desire to stay on the pitch swayed the decision to keep him on. Feyi-Waboso is studying medicine.
"It's getting the balance right, because the player is telling you he wants to try and run it off,"Sinfield said.
England have the final say over the fitness and conditioning of centrally-contracted players such as Feyi-Waboso. There have been disagreements in the past with Exeter over his treatment.
Feyi-Waboso has scored nine tries in 17 appearances for England since making his Test debut against Italy in 2024.
Sinfield was knighted last month for his services to both codes of rugby and for raising money and awareness about motor neurone disease.
"It's been a great journey and it's important we continue because there's no cure,"Sinfield said.