Gallinari was injured playing for Italy in a FIBA World Cup qualifying game against Georgia on Saturday. The Italian national team later announced the veteran sharpshooter sustained a torn meniscus, without providing further details.
ESPN reported on Friday that Gallinari was hopeful of returning late in the 2022-23 season. Typical recovery time for an ACL tear is anywhere from six to 12 months.
The Celtics confirmed on Twitter on Friday: "Danilo Gallinari has been diagnosed with a torn ACL in his left knee. Gallinari sustained the injury while playing for his home nation of Italy in a FIBA World Cup qualifier against Georgia on August 27. Further updates will be provided as appropriate."
Gallinari previously tore the ACL in the same knee while playing for the Denver Nuggets late in the 2012-13 season. That injury required an additional surgery that sidelined him for the entire 2013-14 campaign.
The 34-year-old signed a two-year, $13.3million contract with Boston in July, just days after being waived by San Antonio. Gallinari was acquired by the Spurs from Atlanta on June 30 as part of the trade that sent All-Star point guard Dejounte Murray to the Hawks.
Gallinari, who averaged 11.7 points and 4.7 rebounds in 66 games with the Hawks last season, was being counted on to provide bench scoring for a Celtics team coming off an appearance in the NBA Finals.
He also took to Twitter on Friday to post a statement, saying: "This has been a tough week for me as I have learned the extent of my injury. This game means everything to me and not being able to be on the court with my Celtics teammates hurts.
"I plan to give everything I can to the Celtics organisation and my teammates as we hunt for a title. I will work tirelessly with the Celtics staff to return to the court as soon as I can and I appreciate the unwavering support from the fans, my teammates, and the entire NBA family."
The 13-year veteran is one of only six players to average at least 15 points per game and shoot 40 per cent or better from three-point range over the last four seasons (min. 100 games), along with 2022 All-Stars Stephen Curry and Karl Anthony-Towns, Kyrie Irving, T.J. Warren and Bojan Bogdanovic.