Al Horford and Celtics agree to two-year, $20 million extension

Al Horford is averaging 10.9 points and connecting on a career-high 48.8 percent of his 3-point attempts.Erin Clark/Globe Staff

The Celtics and veteran forward Al Horford have agreed on a two-year, $20 million contract extension, league sources confirmed Thursday. According to a source, the deal is fully guaranteed, with a trade kicker (a bonus the player receives if he is traded).

Horford, 36, is making $26.5 million this season in the final year of the four-year, $109 million deal he signed with the 76ers after leaving the Celtics in free agency in 2019.

Horford is averaging 10.9 points, 6.3 rebounds, and 2.8 assists per game while connecting on a career-high 48.8 percent of his 3-point attempts. Horford has helped the Celtics roar to an 18-4 start while steadying the interior as center Robert Williams remains sidelined following offseason knee surgery.

Horford, a five-time All-Star, has made it clear that winning a championship is his only remaining goal, and sources said he accepted a below-market deal with the Celtics because he believes this is his best opportunity to do that.

In 2016, Horford left the Hawks and signed a four-year, $113 million contract with the Celtics, one of the biggest free agent coups in the franchise’s history. But after the Celtics lost to LeBron James and the Cavaliers in the Eastern Conference finals in consecutive seasons, they were swept by the Bucks in the 2019 semifinals, wrapping up a tumultuous year that turned out to be Kyrie Irving’s last one in Boston.

That summer, Horford opted out of the final year of his deal with the Celtics and joined one of the team’s biggest rivals, the 76ers. In addition to being lured by the hefty contract, Horford believed the 76ers and their All-Star duo of Joel Embiid and Ben Simmons presented a clearer path to a title.

But Horford was a clunky fit alongside Embiid, the talented 7-foot center, and by season’s end his role had been reduced considerably. That summer, the 76ers traded Horford to the Thunder in a salary-clearing move.

Oklahoma City was in the midst of a youth movement, and less than halfway through the season, Horford was removed from the rotation and given more of a mentoring role. He has said sitting out the remainder of that season rejuvenated him and allowed him to focus on his personal training.

In the summer of 2021, Celtics president of basketball operations Brad Stevens was seeking to shed point guard Kemba Walker’s maximum-salary deal, and he eventually sent Walker to the Thunder along with a first-round draft pick in exchange for Horford, who was coached by Stevens during his first stint in Boston.

Unlike his predecessor, Danny Ainge, Stevens has shown a willingness to hand out contract extensions. Jayson Tatum and Jaylen Brown were both signed to rookie-scale extensions, and last year Williams agreed to one as well.

Despite Williams’s injury history, the 25-year-old big man’s four-year, $54 million extension is viewed as a bargain. Next year, the starting frontcourt of Williams and Horford will make about $21 million combined. To put that figure in perspective, Suns center Deandre Ayton is slated to make $32.5 million next season.

The Celtics, who are the favorites to win the NBA title this season, have almost their entire core under contract for next year. The lone exception is forward Grant Williams, who was unable to agree on an extension before the October deadline and will become a restricted free agent in the summer.

League sources have said the Celtics plan to keep Williams in the fold, and if the sides are unable to reach a deal, the team would likely match any offer sheet that Williams signs with another team anyway.

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The Celtics on Thursday were recognized for their scorching start, as Jayson Tatum and Joe Mazzulla were named Eastern Conference player and coach of the month for October/November. (These monthly awards are condensed because the season started in mid-October.)

Tatum is averaging 31.6 points, 7.8 rebounds, and 4.5 assists while shooting a career-best 48.8 percent from the field. The star forward erupted for a season-high 49 points in the win over the Heat Wednesday night. The Celtics have outscored opponents by 12.7 points per 100 possessions with Tatum on the court.

Mazzulla, who was named interim coach following the one-year suspension of Ime Udoka, has guided the Celtics to the league’s best record, highlighted by last month’s nine-game winning streak.