Following three collegiate seasons at Tennessee, Alexander entered the 2019 NBA Draft. After going unselected, he signed a contract with the Heat. He spent most of his time in the G League with the Sioux Falls Skyforce. There, he averaged 10.6 points, 9.0 rebounds and 2.0 blocks. He shot 69.2 percent from the field and 71.4 percent from the free-throw line. In the G League ranks, the rookie was sixth in field-goal percentage, fifth in blocks per game, second in true shooting percentage (75.3), fourth in offensive rating (132.3) and fourth in win shares per 48 minutes (.198). Alexander made his NBA debut Aug. 6 against the Bucks in the NBA bubble. He played one minute and grabbed one rebound. His next appearance was the last seeding game, where Alexander recorded two points and two rebounds in 12 minutes.