The NFL's Sunday coronavirus scare: What happens if it occurs during the 2020 season?

Matt Nagy's phone buzzed Sunday at 2:51 a.m. For an NFL coach, a call at that hour means trouble. Bracing for the worst, Nagy listened as Andre Tucker, the Chicago Bears' infection control officer, told him that nine members of the organization had tested positive for COVID-19.

Nagy was floored, he told reporters Sunday afternoon. Like the rest of the NFL, the Bears had sailed through the first month of training camp amid the coronavirus pandemic. Not a single Bears player had returned a positive test since the initial intake process. And out of nowhere, it appeared, the team had the kind of outbreak that could disrupt it for weeks and call into question whether the league could truly make it through a 16-game regular season.

"My initial, five-second gut reaction when I was told was just pure disappointment and frustration," Nagy said. "It's hard when you hear that, because everyone is doing such a great job of doing everything that we possibly can in our control to prevent stuff like this.

"When you hear this, you think, 'Wow, what's next?'"

Unknown to anyone at the time, 10 other teams across the league were experiencing similar surprises in what would soon push the term "false positive" atop the list of NFL worries. It didn't take the NFL's health and safety team long to trace the culprit: A lab in New Jersey had returned dozens of false-positive results, including 12 for the Minnesota Vikings and 10 for the New York Jets. A total of 77 league employees returned positive tests; all came from the New Jersey lab.

According to a source, all 77 individuals were retested using the original sample. All came back negative. Each of the 77 also took new point-of-care tests. All came back negative. So in all likelihood, it appears the NFL was gifted a consequence-free moment to identify a potential vulnerability in its testing procedures as well as individual teams' reactions to disastrous news.

"Honestly for us, it's probably good that it happened now," Vikings coach Mike Zimmer said. "Because we were able to adjust and adapt and figure out the kinds of things that would happen if it did happen during the season and what we would do from there."

The league said it was working with its testing company, BioReference, to investigate the lab issue. Neither offered a public explanation on Sunday. But it's clear that false-positive test results now rank as the most vexing issue facing the NFL's test protocols, especially given the impact they could have on a game.

The league has already intervened once to strengthen its protocols after Detroit Lions quarterback Matthew Stafford was forced onto the COVID-19 list by what was later determined to be a false-positive test result. Now, every positive test result is followed by two more tests -- one a traditional polymerase chain reaction (PCR) and the other a rapid-response -- in a 24-hour period. If both return negative, the individual is reinstated to the team facility.

That approach might not hold up during the regular season, however. What would happen if a player or coach tests positive on the day before a game? Would he or she have enough time to pass through the ensuing two-test process before kickoff? The league wasn't prepared to answer that question on Sunday, and it is still working with the NFL Players Association on regular-season game-day testing protocols. One option is to cut off testing 48 hours prior to a game, to provide enough time to resolve any lingering questions.

Weeding out false positives also is important for contact-tracing procedures designed to minimize the spread of infection. The NFL's protocol triggers a digital review of proximity-tracking data every time a positive test is recorded. Anyone who was within six feet of a person who tested positive, for at least 10 minutes, during the previous 48 hours is required to isolate. That list would get pretty long for any team that practiced during that time period, but a confirmed false-positive would eliminate the need to isolate.

Before learning about the lab irregularity, Buffalo Bills general manager Brandon Beane didn't think his team would have enough people to practice Sunday morning. The team already knew it would hold out the players who had tested positive, including quarterback Josh Allen. But Beane didn't take for granted that their teammates would want to practice amid the surprise and confusion. So he and Bills coach Sean McDermott set up a conference call for the players with NFL chief medical officer Allen Sills to explain the lab irregularity and answer their questions.

"I think that was good for them to hear," Beane said. "Because naturally, if you're going to be out there tackling and blocking, and passing the ball around ... people might be a little unnerved if we've got some guys missing. We want to be very transparent with our players. We want to be fair to everyone. It's not just them. People are going home. We're not in a bubble, and we want to make sure we're doing this right."

In Chicago, Nagy spent a good portion of his morning explaining the episode to coaches and players in individual conversations. He then held a Zoom call with more than 140 members of the organization to discuss the decision to practice Sunday afternoon.

"We keep talking about trust," Nagy said. "... If you trust us, and you realize that we will never put you or your family in harm's way, then let's go practice and let's go do our deal."

Assuming the lab issue is correctable and short-lived, the NFL has ended its weekend in the same place it began it. There are only three active coronavirus cases among its 2,500 players. A total of 111 players have been placed on the COVID-19 list over the past 28 days, and that includes those who merely had close contact with someone who tested positive. A handful of coaches -- including Doug Pederson of the Philadelphia Eagles, Anthony Lynn of the Los Angeles Chargers and Sean Payton of the New Orleans Saints -- have been affected, but no team has experienced a documented outbreak. In the end, Sunday's episode might soon come to be viewed as a net positive.

"I'm sure they'll get to the bottom of what happened," Beane said. "Now, if it was happening a lot, I think that would start to shake anyone's confidence. But we've been using [the tests] for a month or so now, and this is the first time this seems to have happened.

"It was a good fire drill for our coaches ... and everyone."

ESPN's Courtney Cronin, Marcel Louis-Jacques and Mike Reiss contributed reporting to this story.