Johnson was diagnosed with a left distal radius fracture after undergoing an MRI on Sunday morning. He was hurt in the second quarter of Atlanta's win over the Washington Wizards on Saturday night and left after playing eight minutes.
Johnson, who turns 22 next month, was enjoying a breakout season. The No. 20 overall pick in the 2021 draft was posting career highs across the board, averaging 14.1 points, 7.3 rebounds and 2.4 assists and shooting 42.5% from the 3-point line.
Johnson has started 12 of 15 games for the Hawks and is averaging 29.9 minutes, double his 14.9-minute average from last season.
Reuters contributed to this report.