Following a dysfunctional 2018-19 season that ended with Kyrie Irving sabotaging the Boston Celtics in a second-round playoff exit and reneging on his commitment to re-sign with the storied franchise, Al Horford chose the security of a more lucrative $109 million offer from the Philadelphia 76ers to leave in free agency.

Two seasons later, the prodigal veteran found his way home to Boston, where the fledgling Celtics he left were preparing to spread their own All-Star wings, and together they delivered what had escaped Horford for 14 seasons and 141 playoff games, more than any other active player: his first NBA Finals appearance.

Horford anchored a dominant defensive effort against the Miami Heat in Game 7 of the Eastern Conference finals on Sunday night, and the three players whose careers he has shepherded in Boston — Jayson Tatum, Jaylen Brown and Marcus Smart — provided enough offense to survive a 100-96 series-clinching victory.

"Nobody deserves it more than this guy on my right right here, man," said Brown, who spent the first three seasons of his career playing alongside Horford and sat beside him on the postgame podium. "His energy, his demeanor, coming in every day, being a professional, taking care of his body, being a leader, I'm proud to be able to share this moment with a veteran, a mentor, a brother, a guy like Al Horford. He's been great all season, really my whole career. I'm happy to be able to share this moment with somebody like him."

Horford, whose maternal grandfather died before Boston's heartbreaking Game 6 loss, fell to his knees as the emotions hit on his first conference finals win in four tries. He hunched over and screamed a single word into the hardwood of FTX Arena. "Yes," he said repeatedly, before teammates helped him to his feet.

"My grandfather was somebody I was extremely close with, somebody I really care for," said Horford, "and all week my mom, my family told me to just go out there and play. That's something that he would have wanted me to do, to just continue on and try and stay focused and understand that he's at peace now."

Horford became the biggest free-agent signing in Celtics history when he signed in 2016. His Atlanta Hawks had just beaten Boston in a first-round series, but they were swept by LeBron James' Cleveland Cavaliers in the conference semifinals, and he saw what the Celtics were building. Boston had Isaiah Thomas, Smart, a hodgepodge of hard workers and the last vestige of the 2008 championship Celtics — two high lottery picks obtained from the Brooklyn Nets in exchange for Paul Pierce and Kevin Garnett.

Horford led the Celtics to the East finals in 2017 and 2018, respectively the rookie years for No. 3 overall picks Brown and Tatum. Boston had traded Thomas for Irving and added Gordon Hayward between those two years, but season-ending injuries to both All-Stars left the Celtics with Brown and Tatum as their leading scorers in a seven-game loss to who else but LeBron's Cavs in the 2018 conference finals.

"When you lose those series, obviously it hurts and it's tough," said Tatum. "But you never forget it. That's what we all have in common. We've all been through those tough times, and we remember how that felt."

All the while, Horford was the consummate professional. He was ridiculed nationally for losing to LeBron in four straight playoffs and locally, where a Boston sports talk radio host nicknamed him "Average Al," ignoring Horford's contributions as a center who allows the Celtics to defend and space the floor offensively from all five positions. They just didn't have the firepower around him to match LeBron's greatness, at least not yet.

So, when James left the East, and Irving undermined the Celtics, Horford faced another difficult career decision between an incumbent team with a sub-championship ceiling and the next young roster on the rise. He chose Joel Embiid, Ben Simmons and the Sixers, only to watch the evolving trio of Tatum, Brown and Smart lead Boston back to the 2020 Eastern Conference finals, where they lost to Miami in six games.

When Horford's frontcourt partnership with Embiid failed in Philadelphia, the five-time All-Star heard boos from Sixers fans and worse from their front office. They traded him, along with a lightly protected first-round pick, to the Oklahoma City Thunder in a salary dump. The Thunder put Horford on ice in their pursuit of a high lottery pick, and his career was headed for an unceremonious end, short of championship aspirations.

Only, the Celtics came calling again, once more needing him to captain a young crew who had fallen off course during a .500 season. Horford was available to anyone who could match his $27 million salary last summer, and nobody rang until Boston offered Kemba Walker and a first-round pick in a deal that was panned by some analytics-driven basketball people. Horford was the same as he ever was, undervalued.

The Celtics started this season on shaky ground, and first-year coach Ime Udoka reminded them of their instability at every downturn. Horford laid the foundation for their defensive identity, and he trusted the offensive development of Tatum, Brown and Smart. As necessary as Udoka's brutally honest criticism was, Boston needed Horford's steady hand to counterbalance the emotions on a young team still finding its way.

"When he came back, that gave us a sense of security," Smart said of Horford. "We've got Al back there. He's always going to make the right play on both ends, he's going to calm us down, he's going to show us what we missed, and he's going to help us learn the game even more. We appreciate everything that he brings to this game — his mentality, his professionalism, and that's the big part for us, the way he comes in every day to work. We look up to that, and we try to implicate that into our lives and into our game."

Their faith in one another fueled a remarkable turnaround. Horford was the eldest in a brotherhood that formed the best team in basketball for the final four months of the regular season. Their bond on defense was as unbreakable as anything the league has seen since the last time the Celtics made the Finals in 2010, and the rising stars were realizing their potential. Tatum made the All-NBA first team, Smart was the Defensive Player of the Year, and Brown received votes for both the All-NBA and All-Defensive teams.

Just as important to Boston's success, Horford was Horford again. Given the chance to resurrect his career with a roster he helped raise, they lifted each other to heights no one imagined a year ago. They outgunned Kevin Durant's Nets, out-dueled Giannis Antetokounmpo's Bucks and, yes, outlasted Jimmy Butler's Heat.

"I've been a part of a lot of great teams, and I'm so proud of this group," said Horford. "I've seen [Brown] come in the league, take steps, take levels. I've seen [Tatum], the same thing. I've seen Smart grow. It's just special to be with them and be able to help them and be a part of this. I'm grateful to be in this position."

All that is left to hoist is the franchise's 18th banner. Only the three-time champion Golden State Warriors stand in the way of Horford's Hall of Fame legacy and fully realized potential for the roster he has mentored.

"Every athlete's dream is to get to that final stage and have an opportunity," said Smart. "I've been here four years in the Eastern Conference finals, and I've been sent home every year. It feels really good, and it feels really good for Jayson and Jaylen. We've been together for the longest — even Al, man. Al, just happy for him. He played all these games, and he works his tail off. He deserves it more than any of us."

We so rarely see a second chance in sports, or ever in life, but these Celtics are making of the most theirs.

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Ben Rohrbach is a staff writer for Yahoo Sports. Have a tip? Email him at rohrbach_ben@yahoo.com or follow him on Twitter! Follow @brohrbach