The NFL's chief medical officer, Dr. Allen Sills, said the NFL would need widespread COVID-19 testing before allowing practices and games to go forward.

"As long as we're still in a place where when a single individual tests positive for the virus that you have to quarantine every single person who was in contact with them in any shape, form or fashion, then I don't think you can begin to think about reopening a team sport," he told Judy Battista of NFL.com. "Because we're going to have positive cases for a very long time."

NFL general counsel Jeff Pash had previously said the goal was to hold a full 16-game season and start on time in September. Sills suggested that while that would be the best possible outcome, it would be premature to assume it will happen. 

"I think what was implied there was to say we are not at a point where we are saying that is absolutely not going to happen so we should continue our planning and preparations as if we're going to be able to do that," he said. "But obviously we're going to have to evaluate that along the way. And follow what the recommendations are from public health officials and from our infectious disease experts and others."

There are a number of questions that major sports across the globe will have to answer before live sports will resume. Sills noted that it's unclear how large gatherings at sporting events will be handled before a coronavirus vaccine is created and circulated. That could lead to games being played at empty arenas and stadiums.

Then there is the time needed for OTAs, training camps and the preseason. If, for instance, the United States is still following social distancing guidelines heading into the summer and restricting large gatherings, that could make holding those offseason staples impossible. That, in turn, could delay the start of the season. 

"There's no way to recreate a four-month offseason program in the span of week or two," he noted. "When you think about resuming something like football, everyone understands there needs to be some time to acclimatize to activity and train back to the level of physical fitness just to be able to think about more football specific work. That's part of the timeline you have to plan for."

Those questions have already made it unclear if leagues like the NBA, NHL or the European soccer leagues will feasibly be able to resume their seasons. Ditto for MLB, which hasn't started its season yet and could be facing a shortened schedule later in the year if it's able to begin play.

The NFL has more time to iron out the details of its season and come up with contingency plans if a normal start date becomes infeasible. But NFL fans may want to plan for the possibility that the season will not begin on time.