Hackett got candid at his meeting with media members less than 24 hours after losing to Seattle ... revealing he should have, in fact, stuck with Russell Wilson on fourth-and-five instead of kicking a 64-yard field goal as the time ticked away on "MNF."

"Looking back at it," he said, "we definitely should have gone for it."

The Broncos were trailing the Seahawks, 17-16, but Wilson, who's no stranger to late-game magic, was driving the Broncos down the field with the hopes of winning in his much-anticipated return to Seattle.

The problem? When Russ' drive stalled near midfield -- Hackett elected to pull him from the game, and trot Brandon McManus to attempt to make a historically long kick.

Of course, he missed ... and the Broncos lost -- and Hackett made it clear he regrets the decision.

"It's just one of those things that's unfortunate, but that was our plan going into it," the rookie head coach said. "Obviously, looking back at it, if you missed the field goal, you're always going to wish you would've gone for it, and if you would've gone for it and not gotten it, you wish you would've given him a chance."

Hackett's decision was so questionable ... Hall of Fame QB Peyton Manning, who was hosting the ManningCast during the game, was visibly dumbfounded over it all.