On June 25, 2015, Harrison was selected with the 44th overall pick by the Phoenix Suns in the 2015 NBA draft . His rights were then traded to the Memphis Grizzlies in exchange for Jon Leuer .  He joined the Grizzlies for the 2015 NBA Summer League and averaged points, rebounds and assists in five games.  On October 31, 2015, he was acquired by the Iowa Energy of the NBA Development League , the affiliate team of the Grizzlies.  He made his professional debut for the Energy on November 14 in a 98–95 win over the Sioux Falls Skyforce , recording 11 points and seven assists in 29 minutes.  On February 16, 2016, he scored a season-high 36 points in a 115–105 loss to the Canton Charge .  In 46 games for Iowa in 2015–16, he averaged points, rebounds, assists and steals per game.