Thomas signed his first NBA contract (an Exhibit 10) with the Rockets in November of 2020, and it was later upgraded to a two-way deal in mid-December. He made his NBA debut Dec. 26 against the Trail Blazers. In 10 minutes, he recorded three assists and three rebounds. He appeared in three more games for the Rockets before being waived. He then signed a two-way deal with the Cavaliers on Feb. 24, 2021. He made his Cavs debut March 12 against the Pelicans, posting three points and one rebound in five minutes. Thomas saw his workload increase dramatically at the end of the year. During his final six appearances, he averaged 8.0 points, 4.7 rebounds, 1.7 assists and 1.2 steals in 28.2 minutes per game. He set single-game career highs of 14 points, 10 rebounds, three assists, three steals and two blocks. Thomas also appeared in 14 G League games between the Canton Charge and Rio Grande Valley Vipers. Overall, he averaged 18.5 points, 7.3 rebounds, 3.5 assists, 1.8 steals and 1.1 blocks. Those numbers earned him a spot on the 2020-21 All-NBA G League Team.