The NFL remains hopeful that its plans for dealing with the novel coronavirus pandemic will enable it to conduct a complete and on-time 2020 season this fall and winter, according to its chief medical officer, despite concerns raised Thursday by Anthony S. Fauci, the nation’s most prominent infectious diseases expert.

President Trump took to Twitter to comment Friday on the remarks made by Fauci, a member of the White House’s coronavirus task force, to CNN, in which Fauci said it might not be possible to hold a football season if players are not isolated from others in a “bubble” environment.

“Tony Fauci has nothing to do with NFL Football,” Trump wrote Friday morning. “They are planning a very safe and controlled opening. However, if they don’t stand for our National Anthem and our Great American Flag, I won’t be watching!!!”

Allen Sills, the NFL’s top medical official, said in a phone interview Thursday evening he is being “realistic” but also remains “optimistic” that the league will be able to begin its season on time in September and play it to completion. Sills expressed respect for Fauci and said Fauci raised relevant issues, but he said he believes the protocols being finalized by the league and the NFL Players Association will be effective.

“I think you certainly can have a set of protocols that apply to all facilities and have a degree of consistency of how you approach the operation of the club, whether that be the layout of the facility, the cleaning and disinfection protocols, the screening and testing,” Sills said. “So that’s what we’re aiming for, is consistency across all 32 clubs. … That’s why I think the catchphrase that we’ve been using is that we have to remain flexible and adaptable.

“We’ll have to continue to work with public health authorities at the local, state and national level to try to keep our eye on what’s happening in individual communities. And I think those protocols will be dynamic. I think that what you see us start out with will very likely change in a number of ways. And that will reflect the fact that there’s new knowledge along the way.”

Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN in comments published Thursday: “Unless players are essentially in a bubble — insulated from the community and they are tested nearly every day — it would be very hard to see how football is able to be played this fall. If there is a second wave, which is certainly a possibility and which would be complicated by the predictable flu season, football may not happen this year.”

NFL officials have been in contact with the White House coronavirus task force, medical experts, governors and other state and local officials, and leaders of other sports leagues.

“When I talk about a bubble, to me what that means is everyone is operating from the same set of guidelines and principles with regard to behavior, with regard to testing and screenings when they’re at both the facility and away from the facility,” Sills said. “We completely support that concept and feel like that is vital for everyone to try to stay healthy through the course of the season.”

The NFL, however, has not focused on a bubble concept like that of the NBA, which is planning to restart its season with all teams gathered at a single location outside Orlando. The NFL plans to have teams practice at their own facilities and play games at their own stadiums, with potential limitations on fans based on local restrictions and, if necessary, games being rescheduled or relocated. But Sills said all options have been discussed with the players’ union.

“We’re looking at any and all scenarios and a number of different principles that we could operate from,” Sills said. “We’ve had discussions around many of those issues. When you have those discussions and when you look at those different models, you’ve got to try to look at the scientific data, the medical underpinnings of what would be required to do that, and also the practical considerations over the course of a six-month season. So those things are going to be very different from one sport to another. The NFL is unique in terms of the pace of our schedule and the way that we prepare and the locations of our clubs.”

The NFL has allowed coaches and other staff members to return to team facilities under strict protocols. It has sent protocols to teams by which players eventually will return to those facilities, although details about testing and treatment remain under discussion with the NFLPA. The NFLPA told agents this week that it expects players to be tested for the coronavirus about three times per week. Most NFL teams are scheduled to open training camp July 28 after teams conducted their offseason programs virtually.

“We have a very simple goal, which is to mitigate risk as much as possible for everyone, whether that’s players, coaches and fans,” Sills said. “I said it’s simple, but it’s not an easy goal. It’s going to depend, again, on a lot of hard work and a lot of attention to detail on a consistent basis. We don’t think the virus is going to disappear. It’s going to remain endemic during the time of what we’re looking at as our season. And as a result, we’re all going to have to remain really vigilant. …

“I think we’ve been very clear throughout that we’re not simply going to just put our heads down and charge forward. We’re going to pay very close attention to what’s happening in each of our markets and work very closely with the public health authorities to make sure that we make the safest possible environment for everyone.”