ALAMEDA (CBS SF/AP) — Raiders star receiver Antonio Brown’s tumultuous pre-season continued Monday with the NFL reiterating its stance that players aren’t allowed to practice or play with unapproved equipment.

Brown had threatened to sit out this season if the league did not allow him to wear his old helmet. On Monday, he took to Twitter to say he was rejoining the Raiders.

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“While I disagree with the arbitrator’s decision, I’m working on getting back to full health and looking forward to rejoining my teammates on the field. I’m excited about this season and appreciate all the concern about my feet.”

Brown has not participated in a full practice for the Raiders after starting training camp on the non-football injury list with injuries to his feet that reportedly came from frostbite suffered while getting cryotherapy treatment in France.

He was cleared to practice on July 28 and participated in part of two sessions but wasn’t around the team last week when he had a grievance hearing with the NFL over his helmet

The league announced in April that the 32 players — including Brown and New England quarterback Tom Brady — who ended last season wearing helmet models not approved by the NFL and the NFL Players Association would not have the option to continuing to wear those helmets this season.

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Brown’s model was discontinued by the manufacturer and it is not certified by the national governing body, the National Operating Committee on Standards for Athletic Equipment (NOCSAE).

NFL spokesman Brian McCarthy tweeted out a statement Monday without mentioning Brown by name that said players can only use helmets that have been certified by experts to be safe to use.

“The player can’t practice or play in games with equipment that’s not approved,” McCarthy wrote. “If he doesn’t play or practice he is in breach of his contract and doesn’t get paid. NFL policy is that helmets have to be certified by NOSCAE. They don’t certify equipment that’s (older) than 10 years.”

The Raiders didn’t practice Monday but are hoping to get Brown back on the field soon.

“The helmet thing is a personal matter to him,” coach Jon Gruden said Saturday. “He has a strong feeling about what he’s worn on his head, and we’re supporting him. We understand the league’s position as well, so we’re in a tough spot.”

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Brown has been the game’s most prolific receiver the past six years but the Raiders were able to acquire him from Pittsburgh for just a third- and fifth-round pick in March because of problems off the field.