Ben Roethlisberger says the Steelers better pay T.J. Watt ASAP ... explaining to reporters on Wednesday a new contract for the defensive star is "one of the reasons I took less money."
"I think T.J. should get whatever the heck he wants," Big Ben said. "He's arguably the best football player in the game right now, not just on defense, just in general."
Watt is currently locked in a heated contract dispute with the Steelers ... and things have gotten so contentious, there's a chance Watt -- who's sat out team drills in Pittsburgh practices while waiting for a new deal -- might miss the season-opener on Sunday over it all.