Josef Martinez tore the anterior cruciate ligament (ACL) in his right knee during Atlanta United's opening game of the MLS season on Saturday.
The Venezuela international was taken off on a stretcher 69 minutes into Atlanta's 2-1 win over expansion franchise Nashville after colliding with David Romney.
Martinez's head coach Frank de Boer admitted after the game the injury "doesn't look really well" and on Sunday the club confirmed the 26-year-old would need surgery on a torn ACL, likely ruling him out for most, if not all, of the 2020 MLS season.
The striker has scored 77 goals in 84 games for Atlanta, including a then-MLS record 35 in 2018, when he was named the league's Most Valuable Player in a campaign that ended with United winning the MLS Cup.
News of Martinez's injury will also come as a blow to Venezuela, who have two World Cup qualifiers later this month and kick off their 2020 Copa America campaign against Brazil on June 13.