Antonio Brown 'Going To Be A Pain In The Ass' In Oakland, Says Jack Del Rio

Sorry Jon Gruden ... Antonio Brown is going to cause MASSIVE headaches in Oakland -- so says Jack Del Rio, who says AB "has clearly shown that he's going to be a pain in the ass."

Of course, Brown's Steelers career was FILLED with drama ... and it got so bad in Pittsburgh, it's the reason he's actually in the silver and black today.

But, the trade from Pitt to the Bay Area hasn't quieted the NFL superstar -- in fact, he's kept up most of his beefs, even going after JuJu Smith-Schuster on social media this month.

Because of that ... Del Rio thinks it won't be smooth sailing with the Raiders, telling NFL.com, "If he continues doing the things he's been doing, just like he upset the Steelers, he's going to upset the next team he's on."

"I mean, this is America and he's free to do what he wants -- but those aren't the things that are indicative of a good teammate."

Del Rio says Brown IS a hard worker and a super talent, so it won't be all bad times in Oakland ... but the ex-head coach says he sees the writing clearly on the wall for the future Hall of Famer.