Raiders general manager Mike Mayock issued an ultimatum to the star wide receiver after he failed to attend the Raiders' workout in Napa, Calif.
"You all know that A.B. is not here today, right?" Mayock told reporters. "So, here's the bottom line. He's upset about the helmet issue. We have supported that, we appreciate that. But at this point, we've pretty much exhausted all avenues of relief.
"So, from our perspective, it's time for him to be all-in or all-out, OK? So, we're hoping he's back soon. We've got 89 guys busting their tails, we are really excited about where this franchise is going and we hope A.B.'s going to be a big part of it, starting Week 1 against Denver."
Brown returned to training camp Tuesday and traveled with the Raiders to Arizona for the team's Thursday night preseason game against the Cardinals. He was also a full participant during Oakland's walk-through Saturday, with the team expecting him to practice fully Sunday.
The former Pittsburgh Steelers wideout returned to camp after finding a newer model of his old helmet, which was no longer certified for use. Brown wasn't able to wear the 10-year-old helmet because NOCSAE won't certify any helmet that is made over 10 years ago.
League sources told ProFootballTalk that NOCSAE certified one specific Schutt Air Advantage helmet, made in 2010, for use, but the NFL told the Raiders that the helmet would require additional testing before it could be used. The league informed Brown that the helmet failed the test.
Brown has also been hampered by frostbitten feet after not wearing proper footwear while using a cryogenic therapy chamber. The All-Pro receiver was acquired this off-season in a trade with the Steelers.