The 2019-20 season marked Watanabe's second year in the league and with the Grizzlies. He appeared in 18 games, averaging 2.0 points and 1.1 rebounds in 5.8 minutes. He managed career highs in both field-goal percentage (44.1) and three-point percentage (37.5). The 6-foot-8 forward scored a season-high nine points against the 76ers on Feb. 7, also accumulating one rebound, one assist and a steal in 12 minutes. Watanabe recorded a season-high three rebounds on Dec. 14 against the Wizards and achieved the feat again Feb. 26 against the Rockets. Against the Rockets, he also racked up a season-high two assists. Ultimately, Watanable spent much of the season in the G League, where he made 22 appearances (all starts) for the Memphis Hustle. In 32.7 minutes per contest, he averaged 17.2 points, 6.0 rebounds, 1.9 assists, 1.0 steals and 1.0 blocks. The George Washington product also shot very efficiently, converting 54.6 percent of his field goals, 36.4 percent of his threes and 81.6 percent of his free throws. For the Hustle, he ranked third in win shares per 48 minutes (.126) of players who saw at least 500 total minutes for the club.

Watanabe split his rookie season between the Grizzlies and their G-League affiliate, Memphis Hustle. In 15 NBA games, the forward would average 2.6 points and 2.1 rebounds. He would see more action during the second half of the year. Watanabe would record his best scoring effort with 10 points at OKC on Feb. 7. He'd also set a season-high of five rebounds in that contest. Watanabe registered two assists on two occasions. He also recorded at least one steal in four different appearances. Watanabe performed well with the Hustle in 33 matchups and would start 32 times. He'd post averages of 14.2 points, 7.2 rebounds, and 2.6 assists. Watanabe also shot 43.6 percent from the field, 33.1 percent from three-point range, and 82.7 percent from the free-throw line.