Jon Gruden Says Antonio Brown's 'All In' After Practice W/ Certified Helmet

Breathe easy, Raiders fans ... Antonio Brown ain't getting the boot from Oakland -- Jon Gruden said the superstar practiced Tuesday with a certified helmet and is now "all in" with the team.

Of course, Raiders GM Mike Mayock was pissed at Brown over the weekend for not practicing while he fights the NFL over helmet issues ... and gave the WR a stern "all-in" or "all-out" ultimatum.

Clearly, AB is choosing the former ... 'cause Gruden said Brown put his problems with his helmet aside and fully practiced with the Raiders.

"He's all in, ready to go," Gruden said after the workout. "That's my understanding. Really happy to have him out here. He's a great player."

Gruden also said it appears Brown has put his frostbitten feet behind him ... saying he's under the impression AB is healthy and ready to go.

But, AB is still holding out hope he can switch back to his old helmet ... a source close to Brown confirmed with TMZ Sports on Monday that Brown has filed a new grievance with the league to try to get his old helmet cleared for at least the 2019 season.

Unclear when an independent arbitrator is set to hear Brown's case vs. the league, but NFL Network's Ian Rapoport reported it's supposed to be "expedited."