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.