On June 25, 2015, Christmas was selected by the Minnesota Timberwolves with the 36th overall pick in the 2015 NBA draft. His rights were then traded to the Cleveland Cavaliers along with those of Cedi Osman and a future second-round pick in exchange for the rights of Tyus Jones. He later joined the Cavaliers for the 2015 NBA Summer League, where he averaged 8.0 points and 4.5 rebounds in four games. On July 23, 2015, his rights were traded again, this time to the Indiana Pacers in exchange for a 2019 second-round pick. Four days later, he signed with the Pacers. Christmas spent the majority of the 2015–16 season on assignment in the NBA Development League with the Fort Wayne Mad Ants. On January 29, 2016, he was named in the East All-Star team for the 2016 NBA D-League All-Star Game. In the Pacers' 2015–16 regular season finale, Christmas made his NBA debut, scoring four points and making both his field-goal tries off the bench in a 97–92 win over the Milwaukee Bucks. He spent more time with the Pacers in 2016–17, appearing in 29 games, but still received multiple assignments to Fort Wayne.