The news is a massive blow for the Galaxy, who are facing their first MLS Cup final in a decade.
Puig, who scored 17 goals and produced 20 assists for the Galaxy in 36 matches this season, suffered the injury during Saturday's 1-0 win over the Seattle Sounders, which earned Galaxy a place in the final.
The Galaxy said that Puig had suffered the injury during the second half of the game but continued to play for another half hour, allowing him to make the match-winning assist for Dejan Joveljic’s 85th minute goal.
Puig was limping and in discomfort at the end of the game and the club said that an MRI scan on Sunday had revealed a "torn anterior cruciate ligament (ACL) in his left knee".
The club said he would undergo surgery on the injury at a date yet to be determined.
The 25-year-old has been fundamental to the Galaxy's return to the pinnacle of MLS after they finished in penultimate place in the Western Conference last season.
A product of Barcelona's famed La Masia academy, he came through the club's youth ranks and made 42 appearances for the first team before he joined the Galaxy in August 2022.
In May, he signed a new contract with the MLS club, keeping him with them until 2027.
