The Celtics have signed center Robert Williams III and guard Josh Richardson to contract extensions, the latest in a flurry of moves by Boston as it tries to shore up its roster before training camp.

"Rob has worked hard to turn himself into a very impactful player, and his talent and selfless play give him an even higher upside to reach going forward," Celtics President of Basketball Operations Brad Stevens said in a statement Tuesday. "Rob has embraced being a Celtic from Day 1. He is a great teammate and is completely committed to getting better. We're excited that he will continue to do so here in Boston."

ESPN reported that Williams' deal is for $54 million over four years. A first-round selection in 2018, Williams appeared in a career-high 52 games with 13 starts last season, averaging a career highs in points (8.0 ppg), rebounds (6.9 rpg), assists (1.8 apg), steals (0.8 spg), blocked shots (1.8 bpg) and minutes (18.9 mpg).

The 23-year-old Williams gives Boston stability in the middle and a coveted rim defender, which he flourished as in his third NBA season.

Acquired in a trade with Dallas last month, the 27-year-old Richardson saw his offensive numbers dip last season with the Mavericks, but he is just two seasons removed from averaging a career-high 16.6 points with Miami in 2018-19.

"We feel fortunate to be able to extend Josh," Stevens said. "He is a talented offensive player, a versatile defender, and plays with an edge and toughness that Boston fans will love."

The deals for Williams and Richardson come days after the Celtics announced a contract extension for veteran guard Marcus Smart, which is for a reported four years and $77 million. The team also signed point guard Dennis Schröder earlier this month to a one-year, $5.9 million deal.

The hope is that all four players, along with the return of big man Al Horford, can help fortify the roster behind All-Stars Jayson Tatum and Jaylen Brown in Ime Udoka's first season taking over for Stevens as coach.

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