Martinez had ACL surgery on the same knee and missed all but one game in 2020 before returning for the next season.
The 2018 MLS MVP has collected two goals and two assists in five matches this season for Atlanta United.
Martinez, 28, has recorded double-digit goal totals in every MLS season in which he has been healthy. He has 91 goals and 14 assists in 113 career appearances (104 starts) with Atlanta since 2017.
Martinez's latest setback adds to Atlanta's injury concerns, having seen offseason addition Ozzie Alonso sidelined last week with a torn ACL in his left knee. Matheus Rossetto, Luiz Araujo and league-record signing Thiago Almada are all nursing hamstring injuries.
Emerson Hyndman, who tore his ACL last season, is still working his way back to fitness.
Information from Reuters was used in this report.