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.