The deal will put Williams, 23, among the top half of centers in the NBA in annual salary and is a bet on him growing into the promise he has shown during the first three years of his career.
Williams has seen his role expand each season, including 13 starts in the 52 games he played last season, when he averaged 8.0 points, 6.9 rebounds, 1.8 assists and 1.8 blocks in 18.9 minutes per game, all career highs.
A career 72% shooter from the field, Williams has shown flashes of the ability to become an impact player at both ends of the court. He's effective as a finisher and a rim protector, and he has also shown impressive passing ability.
What have held him back are the nagging injuries that have limited him to 113 games across his first three seasons after being the 27th overall pick out of Texas A&M in 2018.
Earlier this week, the Celtics came to an agreement with guard Marcus Smart on a four-year, $77 million extension, as Brad Stevens continues to put his stamp on Boston's roster after moving from coach to president of basketball operations after the season.
Boston also traded for guard Josh Richardson, swapped Kemba Walker and a first-round pick for former Celtics center Al Horford, and signed guard Dennis Schroder and center Enes Kanter to one-year deals.
Stevens also worked out a deal with the New York Knicks to turn Evan Fournier's departure into a large trade exception, giving Boston some flexibility moving forward. The Celtics have prevented themselves from having significant cap space next summer by virtue of the Smart and Williams extensions.
"You just try to create different avenues to give yourself options," Stevens said earlier this week. "That's the bottom line.
"This is complicated stuff from a numbers standpoint, but I think we do have some flexibility and some options, which is good. And we've got good players, so we've got a good foundation, and that's exciting as well."