"It's a wonderful opportunity with a historic franchise," Butler President James M. Danko said. "We have done everything we possibly can to keep him. Brad is a very bright, very articulate, and a wonderful, wonderful person who's handled this as well as he's handled everything else you've seen him do."
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. Butler just joined the Big East, but Stevens won't be going with them.
"I didn't treat it as inevitable (that he would leave)," said Collier, who noted that the school had signed Stevens through 2025. "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 leave for 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.
"Our family is thrilled for the opportunity given to us by the leadership of the Boston Celtics, but it is emotional to leave a place that we have called home for the past 13 years," Stevens said in a release issued by the university. "We truly love Butler University and Indianapolis, and are very thankful to have had the opportunity to celebrate so many wonderful things together."
At Butler, Stevens was 166-49 -- the most wins for any Division I coach in the first six years of his career. In 2009-10, the Bulldogs posted the Horizon League's first 18-0 conference record, a 25-game winning streak and an appearance in the NCAA title game, where they lost to Duke 61-59 when a last-second, half-court shot bounced off the backboard and rim.
"Brad has given his talent to our university with exceptional generosity, integrity, and humility," Danko said, calling Stevens "a beloved member of our community."
"We have done everything we can to keep Brad here at Butler; however, the Celtics team has offered Brad and his family a unique opportunity with which no university can compete."
AP Sports Writer Michael Marot contributed to this story from Indianapolis.