Following three collegiate seasons at Nebraska, Roby was selected with the 45th overall pick in the 2019 NBA Draft. He began the season as a member of the Mavericks after being traded by the Pistons, the team that drafted him. However, he did not make his NBA debut with Dallas. On Jan. 24, Roby was traded from the Mavericks to the Thunder for Justin Patton. Roby made his NBA debut with the Thunder on Jan. 29 in a win over the Kings, playing two minutes. He made two more appearances during the season. First, he saw three minutes and posted one rebound against the Spurs on Feb. 23. Next, he saw six minutes against the Bucks on Feb. 28 and posted one rebound. Ultimately, most of Roby's rookie season was spent in the G League between the Texas Legends and the Oklahoma City Blue. With the Legends, he averaged 9.2 points, 7.0 rebounds, 1.4 assists, 1.1 steals and 1.0 blocks. With the Blue, he averaged 6.9 points, 4.8 rebounds, 1.6 blocks, 1.3 steals and 1.3 assists. Overall, he shot 50.0 percent from the field and 40.0 percent from three.