Steelers head coach Mike Tomlin called the Patriots a bunch of "assholes" during a locker room speech to his team ... a speech he didn't know Antonio Brown was streaming out on Facebook Live!!!
The speech came moments after Pittsburgh beat up on Kansas City ... earning a spot to play Tom Brady and company in the AFC Championship next week.
Brown flipped on his live stream to share the moment with his fans ... when Tomlin made the comment that's sure to make the Pats' bulletin board.
Tomlin noted that the Pats got a "day-and-a-half" head start on prep and recovery since they played on Saturday and the Steelers didn't play until Sunday night.
Tomlin's final message to his team ... be cool and stay off social media.