Tony Kornheiser and Frank Isola share their opinions on Antonio Brown refusing to play football for the Raiders without his old helmet. (1:02)

Oakland Raiders receiver Antonio Brown is threatening to leave football forever, reports ESPN's Adam Schefter, because his helmet of choice is prohibited for use in the NFL.

Yes, that's a real sentence about a real thing that's happening right now. Let's dive in.

This can't be real.

Oh, but it is. Brown participated in a two-hour conference call on Friday, Schefter reported, as part of a grievance that he hopes will provide an exception to wear his old helmet. That model is a Schutt Air Advantage, according to a league source, and it was discontinued by the manufacturer earlier this decade.

Why won't the NFL allow him to wear it?

The NFL has a longstanding policy that requires all players to wear helmets that have been certified by the National Athletic Equipment Reconditioners Association (NAERA). And NAERA won't certify the helmet because it is more than 10 years old, according to the league source. Like all products and technology, helmets are frequently updated over time. Every player is made aware of the policy, so Brown should not have been surprised that it would be disallowed in 2019.

Editor's note: A league source said last week, and an NFL spokesman said Monday morning, that the National Operating Committee on Standards for Athletic Equipment (NOCSAE) would not certify Brown's helmet because it was more than 10 years old. In fact, NOCSAE licenses the certification process to the National Athletic Equipment Reconditioners Association (NAERA), and NAERA is the group that does not certify helmets that are more than 10 years old.

What is he supposed to wear instead?

In April, the NFL and NFL Players Association released an updated evaluation of 40 helmet models, 34 of which are approved for use in 2019. Half of the approved models are manufactured by Schutt. The Air Advantage is not among those listed.

The Raiders have been sending Antonio Brown other approved helmets to wear but, at this time, he is not interested in wearing any of them, according to sources. Photo by Kirby Lee/USA TODAY Sports

Who cares? Helmets are helmets, right?

Many players are attached to particular helmet models. Some feel they have been well-protected and don't want to try something different. Others make decisions based on aesthetics. And some try new models and can't get used to them. According to Schefter, Brown believes that his best alternative protrudes and interferes with his vision.

But to never play again because of it?

Yes, that's an extreme reaction -- even for a player of Brown's history. There's no sense trying to explain that one.

What would happen if he wears the old one anyway?

In April, the NFL announced a related but separate policy that banned certain helmets that performed poorly in laboratory testing. The onus was put on teams to ensure that no player had access to or wore a banned helmet. If a team was found to have known about a player wearing a banned helmet, or to have facilitated the use of one, it would be subject to league discipline, an NFL executive said at the time.

Is that why Tom Brady changed helmets this season?

Yes. Brady was able to wear his old helmet for one extra season in 2018, as part of a grandfathered NFL policy, but he has moved to an approved model for this season.

play 1:02 King: AB's immaturity is on full display with helmet demand Peter King summarizes Antonio Brown's helmet issue with the NFL as nonsensical and a lack of maturity.

Would Brown be disciplined?

Not by the NFL, it appears. But the Raiders could address it through team discipline if they chose. And because the NFL policy prevents Brown from practicing or playing in the old helmet, he would be in breach of his contract and be subject to losing that week's game check.

Does Brown have any chance of winning the grievance?

It's difficult to see how, as long as the NFL policy is to allow only NAERA-approved helmets and NAERA won't certify Brown's helmet. Allowing a player to use an uncertified helmet, one that presumably protects him less effectively than more updated models, would strike against the NFL's decision to make helmet choice a primary tenet of its health and safety platform.

What would happen if Brown actually retired?

From a financial standpoint, Brown would walk away from at least $30.125 million that is fully guaranteed in his contract for the next two seasons. The Raiders would absorb about the same in a dead money charge. But it's possible the Raiders could simply place him on a reserve list that would absolve them from having to pay out his contract, while maintaining his $14.9 million cap charge for 2019.