Williams' second NBA season saw him grow into a bigger role with the Celtics, as he gained more court time and increased his production. Though a midseason hip injury cost him playing time, he ultimately appeared in 29 games (one start) and more than doubled his scoring average from 2.5 to 5.2, adding 4.4 rebounds and 1.2 blocks per game. Williams' minutes per game increased from 8.8 his rookie season to 13.4 in 2019-20. His per-36-minute stats were eye-popping -- 13.9 points, 11.9 rebounds, 3.2 blocks and 2.0 steals -- and he posted a 72.7 field-goal percentage, a two-tick jump from his already remarkable rookie figure of 70.6. On Nov. 9 against the Spurs, Williams picked up 11 points -- a career best at the time -- along with a career-high six blocks. In his return from injury, Mar. 3 against Brooklyn, Williams picked up 10 points in less than 10 minutes of court time. He saved his best work for seeding play in the Orlando bubble, though, as Williams shot 7-of-7 from the field for a career-high 18 points along with five boards, three blocks and a steal in another game against the Nets on Aug. 5. The 22-year-old center went on to appear in 13 of Boston's 17 postseason games. In Games 1 and 2 of the second round against Toronto, he scored 10 and 11 points, respectively, helping the Celtics build an early series lead. In Game 3 of the series, Williams grabbed a career-playoff-high seven boards in just under 14 minutes of run.

Williams was a first-round pick by Boston in 2018, getting selected 27th overall out of Texas A&M. Some knee tendinitis caused the Celtics to ease Williams into their rotation, and back soreness as well as some time in the G League ultimately limited him to 32 NBA games in his rookie year. Though his opportunities were limited, Williams showed the ability to take advantage and impact the game as a rebounder and shot blocker. On Dec. 10, he collected 11 rebounds and three blocks against the Pelicans. The month of December also saw Williams record a pair of five-block efforts. He collected a season-high eight points in one of those games, shooting 4-for-4 and adding eight boards in a Dec. 19 effort against Phoenix. Such effective shot selection was typical for him, as Williams delivered an incredible 70.6 field-goal percentage on the season, though his limited opportunities held him to 2.5 points and 2.5 rebounds per game -- along with an impressive 1.3 blocks. Though he was lightly used during the Celtics' nine-game playoff run, Williams contributed when given the opportunity. He played nearly nine minutes in Game 2 of Boston's second-round series against the Bucks, delivering four points and five rebounds.