Harper played in three games for Phoenix during the conronavirus-shortened 2019-20 season. He made his NBA debut on opening night, Oct. 23, with a brief appearance during a win over the Kings. Harper logged eight total minutes across three games with the Suns. He spent most of his rookie campaign in the G League with the Northern Arizona Suns. With Northern Arizona, the young point guard averaged 20.2 points on 15.3 shots, 5.5 assists, 2.6 rebounds and 1.1 steals in 28.9 minutes per contest. Harper was waived by the Suns on March 12th, just after the sports stoppage. Due to the pandemic paused season, the Knicks had to wait until June 25, at which time New York picked up Harper and signed him to a two-way contract.