Dr. Anthony Fauci believes it is feasible for the NFL to play games with fans in 2020, despite labelling the sport the "perfect set-up for spreading" coronavirus.
The NFL last week released their schedule for 2020 and has not made public any potential contingency plans for the campaign being impacted by the pandemic.
All major sport in the United States is on hold, with the start of MLB delayed and the NBA and NHL seasons derailed by the spread of the virus.
The NFL cancelled public events in Las Vegas for the draft last month, instead holding the event virtually and earning widespread praise for doing so successfully.
There is some scepticism about the league's ability to press ahead with the season, due to start on September 10, as normal.
However, Dr. Fauci - the director of the National Institute of Allergy and Infectious Diseases - and a key member of president Donald Trump's team leading the fight against coronavirus - can envision a scenario where fans are able to attend games.
"I think it's feasible that negative-testing players could play to an empty stadium," Fauci told Peter King of NBC Sports. "Is it guaranteed? No way. There will be a virus out there and you will know your players are negative at the time they step onto the field. You're not endangering.
"Also, if the virus is so low that even in the general community the risk is low, then I could see filling a third of the stadium or half the stadium so people could be six feet apart.
"I mean, that's something that is again feasible depending on the level of infection. I keep getting back to that: It's going to depend. Like, right now, if you fast forward, and it is now September. The season starts. I say you can't have a season, it's impossible. There's too much infection out there. It doesn't matter what you do. But I would hope that by the time you get to September it's not gonna be the way it is right now.
"If I test today, and I'm negative, you don't know if I got exposed tomorrow. There's no guarantee that you're going to get exposed and be positive the next day. To be 100 per cent sure, you've got to test every day. But that's not practical and that’s never going to happen. But you can diminish dramatically by testing everybody Saturday night, Sunday morning, and say, 'Okay, only negative players play.'"
On how transmissible the virus would be during NFL games, Fauci said: "Sweat does not do it. This is a respiratory virus, so it's going to be spread by shedding virus. The problem with virus shedding is that if I have it in my nasal pharynx, and it sheds and I wipe my hand against my nose - now it's on my hand.
"You see, then I touch my chest or my thigh, then it's on my chest or my thigh for at least a few hours. Sweat as such won't transmit it. But if people are in such close contact as football players are on every single play, then that's the perfect set-up for spreading.
"I would think that if there is an infected football player on the field - a middle linebacker, a tackle, whoever it is it - as soon as they hit the next guy, the chances are that they will be shedding virus all over that person.
"If you really want to be in a situation where you want to be absolutely certain, you'd test all the players before the game. And you say, 'Those who are infected: Sorry, you're sidelined. Those who are free: Get in there and play.'"