The 36-year-old jumped to receive a post-entry pass and planted his left leg awkwardly, with his knee appearing to bend inwards.
Butler yelled in pain and appeared to be unable to put weight on the leg as he was helped off the court and to the locker room by teammates.
He had 17 points, four assists and two steals in the game before suffering the injury.
The six-time all-star was averaging 20.1 points, 5.6 rebounds, 4.9 assists and 1.4 steals entering Monday’s contest.
Butler was traded from the Heat to the Warriors at last season’s trade deadline in a five-team deal that also sent Canadian Andrew Wiggins to South Beach.
The Warriors had won five of their past six games and are eighth in the Western Conference with a 24-19 record.
The Toronto Raptors face the Warriors on Tuesday (Sportsnet ONE, Sportsnet+, 10 p.m. ET / 7 p.m. PT).
