The problem with these Tom Brady-Peyton Manning showdowns is that we’ve become so obsessed with the history that we feel the need to write it before it even happens.

As in, “Manning needs to win this game more than Brady does, because he’s only won one Super Bowl and if he doesn’t win on Sunday his legacy is going to be, blah, blah, blah . . .”

Or: “True, Brady’s won three Super Bowls. But he hasn’t won one in nine years and if he doesn’t win this year, his legacy is going to be, blah, blah, blah.”

Hence the made-up name of tomorrow’s AFC Championship Game between Brady’s New England Patriots and Manning’s Denver Broncos: “The Legacy Game.” (Or “Legacy Bowl.” Whatever.)

How about we let the two teams play and then we’ll write the history?

Here’s a news flash for you: The history we write today isn’t necessarily the history people will be reading — and writing — in 50 years. Consider that Ted Williams was vilified by the press and disliked by some fans well into his career, but that, late in life, he was a revered figure any time he stepped into Fenway Park. Or that Harry Truman’s approval rating when he departed the White House was one of the lowest in history, but that today he is regarded as one of our greatest presidents.

How about movies? Alfred Hitchcock’s “Vertigo” received mixed reviews when it opened in 1958. Today, it’s considered one of the greatest movies ever made — No. 9 on the American Film Institute’s Top 100.

If the AFI had a Top 100 of NFL quarterbacks, where would Brady and Manning rate? Better yet, we don’t need the AFI, because the Internet overflows with quarterback ratings, and most of them place Joe Montana at No. 1, Brady at No. 2, followed by, in no particular order, the usual suspects: Johnny Unitas, Brett Favre, Dan Marino, John Elway . . . and Peyton Manning.

If the Pats win another Super Bowl, does Brady move into a tie with Montana, who also won four Big Games? Does he drop? Does a victory by the Broncos, followed by a victory in the Super Bowl, move Manning up a few notches?

And then there’s the matter of popularity: We now have a poll designed to identify the “least-liked” quarterback in the NFL, and to nobody’s surprise the winner (loser?) is Tom Brady. And why not: He’s rich, handsome, has a supermodel wife and almost always wins.

And while we’re at it, there was apparently some kind of midseason players poll that revealed Bill Belichick to be the second “least-liked coach to play for,” or something like that. The coach was asked yesterday about this, with the question covered up with a heavy dose of syrup, as in: “A midseason poll of players, showed you as the second least-liked coach to play for. That surprised me given your success. Are you surprised that NFL players wouldn’t want to play for you?”

Belichick: “I’m sure everybody has their own opinions out there so whatever they are, they are.”

Return volley: “It suggested they liked your success but didn’t like your hard-nosed approach. I would think success would be the No. 1 goal.”

(That’s a nice little touch there, that “I-would-think-success-would-be-the-No.-1-goal” business. This way, you get the perceived benefit of distancing yourself from the icky poll while mining the ore of Belichick’s dry response.)

Just a guess, but a poll of major league baseball players in 1912 would have revealed John McGraw as the least-popular manager to play for. In football, Vince Lombardi, George Halas and Paul Brown might have inspired similar responses. Today, we refer to these men as “the greatest” without getting too caught up in their people skills.

Anyway, Peyton Manning’s invitation to participate in the poll apparently blew off his front porch.

“Coach Belichick is the best coach that I’ve ever competed against,” he told reporters Wednesday. “I think it’s safe to say he’ll go down as the greatest NFL coach of all time.”

But if the Patriots lose tomorrow, does Belichick drop in Manning’s list? Among football types who keep hammering away about how the Pats haven’t won a Super Bowl since Spygate, does a New England victory tomorrow, followed by a win at the Meadowlands, move Belichick up on the list?

Do we even need tomorrow’s game? We already have the pre-written history. We already have the popularity polls. Why risk screwing everything up with a football game?