After being selected with the 21st pick in the NBA draft, Allen joined the Utah Jazz. He played in 38 games and made two starts during the season. Allen averaged 5.6 points per game during his rookie campaign. He scored a career-high 30 points against the Clippers on Apr. 10. The same game also saw him set highs for rebounds (seven), made three-point field goals (five), made field goals (11) and minutes played (40:43). Allen finished with double digit points in four other games. As a rookie, he spent time in the G-League with the Salt Lake City Stars. At that level, he scored 16.3 points per game and made 12 starts. In the postseason, Allen played in two games in the series against the Houston Rockets. And in the second game, he went on to notch nine points.
June 21, 2018Drafted by the Utah Jazz in the 1st round (21st pick) of the 2018 NBA Draft.
July 2, 2018Signed a rookie two-year contract with the Utah Jazz
July 6, 2019Traded by the Utah Jazz with Darius Bazley, Jae Crowder, Kyle Korver and a 2020 1st round draft pick to the Memphis Grizzlies for Mike Conley.