Two weeks after replacing Danny Ainge as president of basketball operations for the Boston Celtics, Brad Stevens made a seismic first move in his new role.

Boston is sending All-Star point guard Kemba Walker, the 16th pick in this year's NBA draft and a 2025 second-round selection to the Oklahoma City Thunder for Al Horford, Moses Brown and a 2023 second-round pick, the teams announced Friday.

It is unusual for any trade to happen before next week's NBA combine, let alone one this significant. But Stevens has a long-standing relationship with Thunder general manager Sam Presti, and the deal accomplished key objectives for both franchises.

"Kemba is a true professional and a great teammate and player," Stevens said in a statement. "I want to thank him for his tremendous impact, and the positive contribution he's made both to the Celtics and the City of Boston."

The Celtics get significant financial flexibility as Stevens tries to retool the roster around young stars Jayson Tatum and Jaylen Brown. Walker is owed roughly $73 million over the next two years, while Horford is owed $53 million -- $41 million guaranteed -- over the same two years.

That flexibility will give Boston the potential capability to re-sign guard Evan Fournier, whom they acquired at this year's trade deadline and who will be an unrestricted free agent this summer, while also reuniting the Celtics with Horford, who spent three years with them before signing his current deal with the Philadelphia 76ers in 2019.

Oklahoma City, on the other hand, is continuing to acquire draft capital as it rebuilds its roster around Shai Gilgeous-Alexander. The Thunder have three first-round picks in this year's draft: their own, Boston's, and either the fifth pick or the 18th pick, depending on whether Oklahoma City gets Houston's top-4-protected first-rounder after Tuesday's NBA draft lottery. The Thunder now have 18 first-round picks over the next seven drafts.

The trade also continues a consistent trend under Presti for Oklahoma City to work with players to find them a suitable new home, something he has done with Chris Paul, Danny Green and now Horford after acquiring all three within the past several months.

Oklahoma City will work with Walker in a similar fashion, sources said.

The move brings an end to a tumultuous two years for Walker, 31, with the Celtics after his celebrated signing in July 2019 to replace Kyrie Irving as the team's starting point guard. After being one of the NBA's most durable players during his eight years in Charlotte to begin his career, missing a total of 35 games, he has missed 45 the past two seasons, including 29 this year alone.

After spending most of the 2020 calendar year battling left knee issues, Walker underwent a 12-week strengthening program in the offseason to try to improve the knee, missing the start of the season, and then was intentionally held out of one half of every back-to-back set. But Walker, who averaged 19.3 points and 4.9 assists in 43 games, still wound up sitting out the final two games of Boston's season -- Games 4 and 5 of the Celtics' first-round playoff exit at the hands of the Brooklyn Nets -- with a bone bruise in that same left knee.

"It's tough," Walker said after the Celtics lost Game 5, in what became his final public comments as a member of the team. "It's been really tough, especially because over the course of my career, I've played so many games when I've been healthy.

"I came to Boston to be a part of those special runs and be a part of high-intensity games and fans going wild, and I wasn't able to be a part of that, unfortunately. Just try to get right. I got to get right."

Horford, who turned 35 earlier this month, played well in 28 games for Oklahoma City, averaging 14.2 points, 6.7 rebounds and 3.4 assists in 27.9 minutes per game, before he and the Thunder mutually agreed to shut him down for the rest of the season to better ensure they could find a suitor for him this offseason. He will likely be Boston's starting center next season.

He thanked the Thunder staff and his teammates in a tweet, adding "While it was short, it was first class all the way and me and my family are extremely thankful and appreciative."

Brown, 21, impressed as a two-way player before signing a multiyear deal with Oklahoma City, averaging 8.6 points and 8.9 rebounds in 43 games -- including going for 21 points and 23 rebounds against the Celtics on March 27 in Oklahoma City.