Oakland Raiders wide receiver Antonio Brown has reportedly filed a grievance against the NFL for banning a helmet he wore throughout his time with the Pittsburgh Steelers since it's no longer certified by the National Operating Committee on Standards for Athletic Equipment (NOCSAE).

ESPN's Adam Schefter reported Friday league sources believe Brown may be sitting out portions of training camp because of the helmet issue in addition to the extreme frostbite he reportedly suffered while using a cryotherapy chamber in France last month.

Schefter also reported Brown had a two-hour meeting with the league about his helmet. He later added Brown has even threatened to quit the NFL if his helmet isn't allowed:

The receiver took part in a two-hour meeting with league officials Friday where he argued his case and a final decision could "come as early as next week," per Schefter.

Although the NFL and the Players Association have been testing helmets for five years, there's no longer a grandfather clause allowing players to keep their preferred headgear if it loses certification.

"The prohibited helmets will not be allowed in the locker room," Jeff Miller, the NFL's executive vice president of health and safety initiatives, told reporters in April. "A player will not be allowed on the field [wearing such a helmet]."

CBS Sports' Jason La Canfora added more insight into Brown's helmet:

New England Patriots quarterback Tom Brady, one of the players who previously used a now-banned helmet, changed to an accepted model during the 2018 season before switching back in November. He told Westwood One Radio's Jim Gray (via WEEI's Ryan Hannable) at the time he hoped to receive clearance for it during the offseason:

"Yeah, I've gone back-and-forth quite a bit. You know, the old one has just treated me so well over the years, so hopefully I can get grandfathered in. That's what I'm going to try to do this offseason, see if I can keep playing with it. I love the way it fits. I love the way it feels. It's been a great helmet for me. So I've just been thinking about it and think that's what works the best for me as we continue on this season."

The Raiders told Schefter they are "aware and supportive of Antonio Brown, but [have] no further comment."

Michael Gehlken of the Las Vegas Review-Journal reported earlier Friday the seven-time Pro Bowl receiver is expected back at team headquarters in the "near future" after missing 10 of 11 camp sessions.

Oakland opens the preseason Saturday night against the Los Angeles Rams.