The Boston Celtics and veteran Al Horford have agreed on a two-year, $20 million contract extension that will span the five-time NBA All-Star's 39th birthday in June 2025, according to ESPN's Adrian Wojnarowski.

Horford is earning $26.5 million this season in the final year of a $109 million contract he signed with the Philadelphia 76ers in 2019. How he returned to Boston, where he reached two Eastern Conference finals in three seasons from 2016-19, is a redemptive story for one of his generation's most underrated players.

Horford, whose defense against Sixers center Joel Embiid keyed Boston's 2018 conference semifinals win over Philadelphia, never found his footing as a frontcourt partner or backup to the All-NBA big man. The Sixers spent a first-round draft pick to send Horford to the tanking Oklahoma City Thunder and shed his salary after just one season. The Thunder shelved Horford after just 28 games during the 2020-21 season.

Looking to shed salary of their own, the Celtics attached a first-round draft pick to the remainder of Kemba Walker's deal in order to reacquire Horford from the Thunder in June 2021. Horford enjoyed a resurgence in Boston, starting all 23 games of their playoff run to the Finals, mostly as a floor-spacing power forward next to All-Defensive center Robert Williams III. Horford has started at center in Williams' absence this season.

"I remember I was driving home with my family from visiting my mom in Atlanta, and we got the call," Horford said from the Finals podium of learning he would get a second chance to compete for a championship in Boston. "We're just all screaming in the car, just really, really excited, really, really grateful. Right away we're just making plans about heading back to Boston, doing the physical, doing all this stuff.

"It was a really happy time for my family at that time. Especially for me, because it's where I wanted to be."

Horford is averaging 10.9 points (on remarkably efficient 56/49/63 shooting splits), 6.3 rebounds and 2.8 assists in 31.3 minutes per game for the Celtics this season. As importantly, the veteran has served as a mentor to Boston's young core of Jayson Tatum, Jaylen Brown and Marcus Smart on their rises to stardom.

"I remember coming in my rookie year and seeing Al Horford," the 24-year-old Tatum said during the Finals of his reunion with Horford. "I've been seeing him play my whole life. ... He's helped me a great deal in the three years that we've played together, and I'm very lucky and fortunate to be able to call Al a teammate."

"Man, we love Al," added Smart. "He's the best vet I've ever had."

All that makes Horford's $10 million salary on his newest contract extension a bargain for the Celtics, whose 18-4 record leads the NBA at the season's quarter mark and makes them championship favorites. A ring and a few more years to pad his résumé might be all he needs to punch his ticket to the Hall of Fame.

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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