The New England Patriots are transitioning from BB to AB – After Belichick.

In a seismic move that seemed almost inconceivable not even a year ago, the six-time Super Bowl champions parted with longtime head coach Bill Belichick, the team confirmed on Thursday, in the aftermath of a 4-13 campaign – his worst in 24 seasons with the organization.

Hired by Pats owner Robert Kraft in 2000 to replace Pete Carroll, Belichick was on the sideline for all of the franchise’s Lombardi Trophy-winning performances and guided New England to three additional Super Sundays. No NFL head coach has won or appeared in more Super Bowls. His 333 wins, including playoffs, rank second all-time in the NFL’s 104-season history, trailing only Hall of Famer Don Shula’s 347.

"Robert and I, after a series of discussions, have mutually agreed to part ways. For me, this is a day of gratitude and celebration," Belichick said at a Thursday news conference, though he took no questions.

"We had a vision of building a championship football team here. That's exceeded my wildest dreams and expectations, the amount of success that we were able to achieve together through a lot of hard work and the contributions of so many people. I'm very proud of that, and I'll always have those great memories.  I'll carry those with me for the rest of my life.

"It's with so many fond memories and thoughts that I think about the Patriots. I will always be a Patriot. I look forward to coming back here. But, at this time, we're going to move on. I look forward, am excited for the future. But, I am always very appreciative of the opportunity here, the support here, and Robert, what you’ve done for me."

Despite Belichick’s incredible résumé, which also includes two rings as defensive coordinator of the New York Giants, his fortunes – and the Patriots' – took a decided turn following legendary quarterback Tom Brady’s decision to leave the team as a free agent following the 2019 season. New England finished last in the AFC East in 2023 for the first time since 2000, Belichick's first in Foxborough. The Pats have only made the postseason once since TB12 bolted and have not won a playoff game since then. A series of questionable personnel and coaching moves over the past four years – all with Belichick's fingerprints on them – have been largely responsible for an offensive spiral that included the second-fewest points scored in the league this season.

But what a ride it was before reaching the end of the line.

"I’m very proud that our partnership lasted for 24 years," said Kraft. "I don’t think in the NFL, there’s been any other partnership that lasted longer and has been as productive as ours."

Picked in the sixth round of the 2000 draft, just months after Belichick’s arrival, Brady, his coach and the organization at large experienced a meteoric rise after the University of Michigan product replaced injured starter Drew Bledsoe in Week 2 of the 2001 campaign and didn’t relinquish the role for the next 19 seasons – even though Belichick was widely criticized at the time for not restoring Bledsoe as the starter once he was well enough to play. But it was the right call, Brady earning the first of five Super Bowl MVP nods following that 2001 magic-carpet ride, when the Patriots shocked the heavily favored St. Louis Rams in Super Bowl 36 to win their first championship.

Brady won a seventh Super Bowl ring – he has more personally than any franchise in the league does collectively – with the Tampa Bay Buccaneers after signing with them in 2020. Since he left Foxborough, the Patriots' lone playoff appearance was a 47-17 loss to the Bills in Buffalo at the end of the 2021 season. With Brady in the lineup, Belichick won 17 AFC East titles. Without him, not one.

And while it would be fair to expect a falloff for any team after losing the quarterback who was a primary driver behind a dynasty, it seemed Belichick egregiously mismanaged the position. After a 7-9 season with Cam Newton in 2020 – while Brady was leading the Bucs to their Super Bowl 55 win – the Patriots drafted Mac Jones in the first round in 2021. Jones responded with a Pro Bowl rookie season that landed New England a wild-card berth. Yet he has regressed since, playing for three different coordinators in his three seasons, Belichick’s handling of his staff in 2022 especially critiqued after Matt Patricia and Joe Judge – neither assistant with a robust offensive background – worked with Jones. Bailey Zappe replaced him as QB1 for the final six weeks of the 2023 season.

And the problems didn’t end there.

While few would ever question Belichick’s chops as a football tactician, he’s been far less adept as the Patriots’ de facto general manager. He’s certainly drafted and acquired dozens of good players, many discarded by other franchises, and typically found ways to maximize their talents and mask their weaknesses. But Brady, widely acknowledged as the greatest quarterback of all time, also covered up a lot. Belichick has always had a spotty-at-best record evaluating wide receivers, especially young ones, and had a lengthy history of missing on second-round draft picks – regardless of position. Aside from linebacker Matt Judon, Belichick has also misappropriated tens of millions in free agency commitments since Brady’s departure – the Patriots arguably their division’s least-talented team the past two seasons.

Belichick, perhaps feeling the pressure of Kraft’s mandate for the Patriots to reclaim their title-contending pedigree, also appeared to break character at the league’s 2023 annual spring meeting – referring reporters to his historic achievements when asked why fans should be optimistic about the upcoming season. It was a jarring about-face for a coach who’d always preached staying focused on the moment, never looking back while not peeking past the next opponent. His remark even drew criticism from former team captain Tedy Bruschi, a member of Belichick’s first three Super Bowl teams and the 2007 squad that remains the only one to post an undefeated regular season since the 1972 Miami Dolphins.But the angst preceding the 2023 season proved justified, ahead of a cellar-dwelling season. Belichick also went 37-45 in his first head coaching stint with the Cleveland Browns from 1991 to '95.

And yet two last-place seasons among 24 in New England alternately serve as a testament to what he’s accomplished, including a sterling 30-12 postseason record for the Patriots. Brady will be a first-ballot Hall of Famer, but Belichick also coached Canton-minted players such as Ty Law and Richard Seymour with Rob Gronkowski (and perhaps others) sure to follow.

Which is to say, despite the post-dynastic downturn in New England, it seems highly likely quite a few of the league’s 31 other clubs will be all too eager to interview Belichick and afford him the opportunity Brady had to win elsewhere – while tracking down Shula’s record.

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Follow USA TODAY Sports' Nate Davis on X, formerly Twitter @ByNateDavis.

This article originally appeared on USA TODAY: Bill Belichick out as Patriots head coach after 24 years leading team