Allen declared for the NBA Draft in 2018 and was selected by the Utah Jazz with the 21st overall pick in the first round. He spent one season with the Jazz before being traded to the Memphis Grizzlies in 2019. Allen showed flashes of his potential with the Grizzlies, contributing as a role player and providing scoring off the bench. In 2021, he was traded to the Milwaukee Bucks, where he continued to develop his game and contribute to the team’s success.