Since taking Butler of the mid-major Horizon League to the national championship game in 2010 and again in '11, Stevens had been courted by Illinois and UCLA, among others.
"But there are some brands in sports, and in the world of basketball the Celtics are one of those," athletic director Barry Collier said, adding that Stevens' contract ran through 2025. "That shows you the faith we had in Brad and the commitment we made to him. ... I didn't treat it as inevitable (that he would leave). I looked at it like every year Brad was our coach, it was another good year for Butler."
Stevens spent seven years as a Butler assistant and the last six years as the head coach, compiling a career winning percentage of .772. He never won fewer than 22 games in a season, and the Bulldogs went 33-5 in 2009-10.
Stevens, who didn't attend the news conference, takes over a team that is rebuilding just three seasons removed from an appearance in the NBA Finals; the Celtics won their unprecedented 17th championship in 2008. But with Garnett and Pierce showing signs of slowing down in this year's playoffs, when Boston was eliminated by the New York Knicks in the first round, Ainge is trying to get younger.
He allowed Rivers to take over the Clippers, extracting a first-round draft choice in return. Amid last week's NBA draft, the Celtics and Nets agreed to a deal that would send Garnett and Pierce to Brooklyn in exchange for a package of players along with three first-round draft picks.
In all, the Celtics have nine first-rounders in the next five years, along with a dynamic but temperamental point guard in Rajon Rondo and talented swingman Jeff Green.
Now, Stevens will be the one to work with those young players.