The NFL’s chief medical officer acknowledged Monday that “this is going to be hard” as the league attempts to fulfill its long-stated goal of having an uninterrupted season amid the novel coronavirus pandemic.

But as teams prepared to open their training camps fully and NFL leaders eyed the MLB outbreak that raised new doubts about the ability of professional sports leagues to operate successfully outside a bubble environment, Allen Sills said he remained hopeful the league’s health protocols will enable the NFL to stage its season.

“I’ve said all along that I remain cautiously optimistic,” Sills said in a phone interview. “And that’s where I personally view this. As I said before, this is going to be hard. There’s no doubt that this is going to be a challenging year for all of us, not just in the NFL but across all elements of our society. And football is no exception. So I think there’s a tremendous amount of work to do. But I know that we have put a tremendous amount of thought and effort and energy into the protocols and keeping things as safe as possible. I know that we’ve got players and coaches and staff that are very committed to make it work. And I know that it’s something that we all hope very much will show how we can positively live with this virus.”

The NFL’s approach is similar to that of MLB, with plans for teams to play games in their home cities and stadiums. That’s why the outbreak of coronavirus cases among Miami Marlins players and coaches, leading to the cancellation of games Monday night in Miami and Philadelphia, raised further questions about the NFL’s ability to make things work.

“I think we’ve said from the start that this is going to be hard,” Sills said. “The NFL in 2020 is not going to look like it has in other years. The covid-19 outbreak is going to continue to be a major challenge to every aspect of American life, and football is no exception. So we’ve said we expect that we’ll have positive cases among our players and personnel and could even have a number on each team. And that’s true as long as the virus remains endemic in society. New cases will kind of be part of that reality.

“But what’s important is that we have protocols in place to identify these cases as quickly as possible and then to take the appropriate action once we identify them to get that individual separated away from the team environment and to get them that treatment and try to prevent an outbreak on the team. None of that has changed as the result of this news. I think it just reflects the fact that this is going to be challenging for all of us.”

Sills declined to specify how many positive cases would lead to the NFL shutting down a team, canceling a game or ceasing leaguewide operations entirely. He said the league must remain flexible in its planning, not answering a question directly about whether it was too late for the NFL to construct a bubble environment for teams at one or two sites like the NBA, NHL and MLS have done.

“I think the two words that I’ve told everyone will characterize our season are flexibility and adaptability,” Sills said. “We absolutely have to remain flexible about everything we’re doing, from the testing cadence to the treatment response to the setup to the schedule. Everything, I think, will have to remain flexible and adaptable based on what we’re learning and what we’re seeing in real time along the way.”

Some players began reporting to NFL training camps last week. Veteran players for most teams are scheduled to report Tuesday.

“Adaptability and flexibility will be needed for the foreseeable future,” NFL Commissioner Roger Goodell wrote in a letter to fans that was released Monday by the league. “After all, even the best game plan changes as new challenges arise.” Goodell wrote that the NFL hopes to have “a healthy and complete 2020 season.”

A player must test negative three times after reporting to camp before even being allowed to enter a team’s facility on Day 5 after arriving. (That was a late switch in the protocols, up from two negative tests.) The league and the NFL Players Association agreed to a testing program by which players will be tested daily for at least the first two weeks of training camps. The testing frequency can be adjusted to every other day after that based on the rate of positive tests.

A player can be disciplined by a team if found to have participated in a high-risk behavior away from the team’s facility that threatens to spread the virus. Teams previously were given treatment procedures for a positive test, among other elaborate protocols. Teams plan to use tracking devices on players and staff members when they’re in team facilities, aiding with distancing measures and contact tracing.

Yet New England Patriots cornerback Jason McCourty said Sunday night on his podcast that he is “very nervous to think there will be a season.” The Minnesota Vikings announced Monday that Eric Sugarman, their head athletic trainer and vice president of sports medicine, tested positive for the coronavirus, along with family members. He is the team’s designated infection control officer.

The NFLPA previously expressed concern about the propriety of opening training camps in “hot spot” cities, with cases of the virus surging in many areas of the country. That led to a conference call in which NFL team doctors told NFLPA representatives they believed it was safe to open camps.

“I think it’s challenging in any area, whether it’s a so-called hot spot or not, because, once again, the virus is endemic all across the country,” Sills said Monday. “So it makes it challenging not only for players but for coaches, staff and everyone who’s living in that environment. And it emphasizes why what we do at our facilities is very important, but also what our players, coaches and staff do away from the facility is incredibly important as well, the choices that they’re making. They’ve got to make good choices, just like all of us do in society — wearing a mask, practicing good hand hygiene, staying away from sick people, avoiding large gatherings. I mean, all of those things will be critical to the success of our season.”

Any potential decisions about shutdowns or cancellations would have to be made on a case-by-case basis, Sills said, in consultation with the NFLPA, infectious-disease experts and public health officials.

“I think you obviously have to deal with each case in a unique manner,” Sills said. “In medicine, we always know there’s a saying that says when you’ve seen one case of something, you’ve seen one case of something. Every case is different. … Given that, it’s very difficult to create an algorithm, if you will.”