The 22nd overall pick in the 2019 draft, Williams played in 69 of 72 regular-season games for the Celtics during his pandemic-shortened rookie campaign, making five starts in the process. The Tennessee product averaged 3.4 points and 2.6 rebounds per game while mostly playing a rotational role off the bench. Williams proved proficient at winning skills that don't show up in the box score. He was excellent at rotating on defense, delivering up-court hockey assists and setting screens for others. In his fifth NBA contest, a Nov. 1 win over the Knicks, Williams cracked double-digit points for the first time, notching 10 points with four boards. After missing his first 25 NBA three-point attempts (through his first 20 games), Williams improved to 33.8 percent shooting from behind the arc with 0.5 treys per game over his last 49 regular season games. His biggest offensive outburst came on Dec. 20 against Detroit, when he shot 8-of-10 from the field en route to 18 points. On Aug. 13 in the Orlando bubble, he pulled down a season-high eight boards against the Wizards, rounding out his sat line with seven points, two assists and a blocked shot. The 6-foo-6 forward blocked two or more shots in eight games. Williams went on to appear as a reserve in all 17 of the Celtics' playoff games during their run to the Eastern Conference finals. In Game 2 of the first round against the Sixers, he delivered nine points, six rebounds, two assists and a block in 20:36 on the court.
June 20, 2019Selected by the Boston Celtics in the first round (22nd overall pick) of the 2019 NBA Draft.
July 11, 2019Signed a two-year, $4.88 million contract with the Boston Celtics.
December 29, 2020Boston Celtics exercised $2.62 million team option for 2021-22.