An old football belief, pronounced often by Bill Belichick, holds that the NFL season begins after Thanksgiving.

Harsher realities come to bear, revealing deep truths.

Which players, you wonder, can cope best with cold weather, pain and fatigue?

Which depth charts run the deepest?

Among coaches, who’s ready to counter opponents having gone to school on months of game film?

Belichick’s teams took to late-season challenges like champion sled dogs in the Yukon.

In January 2019, the best Chargers team of the past 15 years went to New England for its 18th game. Mild New England weather couldn’t prevent the L.A. newbies from being dismantled. The Patriots won the chess matches, hit harder and erred far less, rolling to a 28-point halftime lead and a four weeks later, a Super Bowl win.

Belichick won eight Super Bowls (two as a Giants coordinator and six at the Patriots’ coach), so let’s assume he’s right in saying that as Americans this week stuffed themselves with turkey, yams and pie, a figurative starter’s whistle blew in the NFL.

Which teams look ready to finish strong?

Not the Miami Dolphins, despite a favorable schedule.

Seeking a final playoff seed, the Dolphins won three games in a row this month.

Then they went to Green Bay for this week’s Thanksgiving night game.

Reminding no one of the crisp, Don Shula-coached Dolphins of the 1970s, Miami looked uncomfortable, undersized and in need of what former Chargers linebacker Shaun Phillips used to call “tough juice.”

“One concern about the Dolphins,” former Dallas Cowboys coach Jason Garrett said late in NBC’s telecast of the Packers’ low-stress victory, “is that they’re not a physical football team.”

Question: if a football team isn’t physical, is it truly a football team?

On the spectrum’s bright end, several teams look stout.

Sunday’s matchup in Baltimore between the Eagles and Ravens pairs physical, large and deep lines, powerful fast running backs in Saquon Barkley and Derrick Henry and quarterbacks and pass-catchers accustomed to cold weather.

Bigger bodies weather the long slog the best, as coaches and scouts have claimed since before the Super Bowl was thought up. If that’s true, the Eagles and Ravens look prepared to go far.

Once again, the Chiefs (11-1) and Bills (9-2) have ideal quarterbacks for the stretch drive.

Patrick Mahomes and Josh Allen can fire strikes through wind and cold. And as the stakes go up, they’re more apt to run for first downs or touchdowns.

Porous at left tackle, the Chiefs will need a quick fix from newcomer D.J. Humphries. The ex-Cardinal joined them this week in his comeback from reconstructive knee surgery. The Chiefs have other flaws, too. But All-Pro right tackle Mitchell Schwartz described them well: “They have an ability to turn up the urgency and tighten the details come the playoffs,” he posted on social media.

The NFC-leading Lions play indoors, accentuating speedsters Jahmyr Gibbs and Jameson Williams. But Dan Campbell’s club might  be more comfortable in snow. Success begins with the offensive line, coached by former University of San Diego assistants Hank Fraley and Steve Oliver. The Lions (11-1) must overcome numerous injuries to their defensive front while trying to repel the Vikings (9-2), Packers (9-3) and Eagles (9-2). Earning the NFC’s first seed would give them the inside track to the franchise’s first Super Bowl.

The Packers, who’ll play at Detroit next week, obtained much-needed physicality from newcomer Josh Jacobs. A running back that Jim Taylor and John Brockington would admire, Jacobs wins most collisions yet has the speed and agility to avoid them. The cold and slop don’t bother Jayden Reed, a stronger-than-he-appears 5-foot-11, 191-pound receiver.

The Vikings look capable of snagging their first playoff win under coach Kevin O’Connell, a Carlsbad native and former San Diego State quarterback. The 39-year-old O’Connell has his third Vikings team among the top-10 in points scored and points allowed. Free-agent quarterback Daniel Jones’ decision to join the Vikings this week reflects well on O’Connell while giving the Vikings a good backup to Sam Darnold.

Thanksgiving is behind us, but for football lovers, the main course and dessert are still to come.