As his career ventured into uncertainty in the years after leaving the Celtics in free agency in the summer of 2019, Al Horford developed a stronger fondness for Boston. Stops in Philadelphia and Oklahoma City drove it home.

“It’s something that I really missed,” Horford said when he was reacquired by the Celtics via trade.

On Thursday, the Celtics ensured Horford will likely never miss Boston again for the rest of his career.

The 36-year-old Horford – who was set to become a free agent after this season – and the Celtics agreed to a two-year, $20 million extension, according to multiple reports. The center’s $26.5 million salary for 2022-23 now becomes fully guaranteed as part of the deal, which will keep him in Boston through the 2024-25 season, when he turns 39.

Horford will now effectively make $46.5 million over the next three seasons, a fair deal for a player who’s become integral to their championship aspirations.

Horford initially signed with the Celtics as a free agent in 2016. While there were questions about how much he could give at this stage of his career when he was reacquired in 2021, Horford has experienced a revival of sorts in his second opportunity.

Horford turned back the clock last spring during the Celtics’ run to the NBA Finals, producing several clutch performances, perhaps none more crucial than Game 4 of the second-round victory over the Bucks, when he scored 30 points on five 3-pointers and a highlight dunk on Giannis Antetokounmpo as Boston evened the series.

The big man’s contributions were all-encompassing. His timely shot-making, versatile defense and steady veteran influence for a young core led by Jayson Tatum and Jaylen Brown have been critical as the Celtics have taken the next step as championship contenders.

This season, Horford has picked up where he left off, as he’s helped the Celtics get off to a league-best 18-4 start in the absence of starting center Rob Williams, who’s expected to return at some point this month.

Horford is averaging 10.9 points per game and shooting a career-best 48.8% from 3-point range. He’s playing 31.3 minutes per game as Williams recovers – his most since 2017-18 – but has been sitting in the second game of back-to-backs in an effort to keep him fresh for a long season.