In November of 2020, Fall signed his second one-year two-way contract with the Celtics. With the Maine Red Claws opting not to play in the G League bubble, Fall spent the entire season with Boston. He appeared in 19 games and averaged 7.2 minutes per contest. Fall averaged 2.5 points, 2.7 rebounds and 1.1 blocks per game. Those blocks project to 5.3 rejections per 36 minutes. Fall made his season debut on Dec. 30 with two points, two blocks and one rebound during a home win over the Grizzlies. On Jan. 8, the 7-foot-5 center played a career-high 19 minutes and generated four points, seven rebounds and three blocks during a home win over the Wizards. On May 5, Fall scored two points and blocked a career-best four shots during a win at Orlando. On the last game of the season, May 16, Fall produced three points, three blocks and a career-high eight rebounds during a road loss to the Knicks. During Boston's five playoff games versus Brooklyn, Fall made one brief appearance in Game 4.2019
Fall went unclaimed in the 2019 NBA Draft, but he signed an Exhibit 10 contract with Boston ahead of the 2019-20 campaign. After a strong training camp, the Celtics decided to upgrade his deal to a two-way contract. Over the course of the pandemic-shortened campaign, Fall went on to appear in seven games for the Celtics and 29 for the G League's Maine Red Claws. The 7-foot-5 center made his NBA debut on Oct. 26, registering four points and three rebounds in a win at New York. He ultimately averaged 3.3 points, 2.1 rebounds and 0.6 blocks across 4.7 minutes per game, teasing impressive potential in a limited sample. With Maine, Fall showed off his strong two-way game, posting averages of 12.9 points, 11.1 rebounds and 2.9 blocks. He scored a season-high 21 points on two occasions for the Red Claws, including a 21-point, 18-rebound effort during a win over the SkyHawks on Dec. 29. At the end of the G League season, Fall was named to the G League All-Defensive Team. In Boston, injuries to Enes Kanter, Vincent Poirier and Robert Williams led to the Celtics carrying Fall on the bench as an insurance policy. The big rookie had his best NBA game so far during seeding play in the Orlando bubble. On Aug. 13, Fall delivered four points, four boards, two blocks and one assist in 8:24 against the Wizards. The lengthy center also made his NBA postseason debut, producing three points and a rebound in three minutes during an Aug. 19 first-round win over the Sixers.