Celtics’ Danilo Gallinari is out indefinitely with a torn ACL

The Celtics signed Danilo Gallinari to provide some bench scoring, but it appears he'll be unavailable for much of the season.Jonathan Wiggs/Globe Staff

Celtics forward Danilo Gallinari has been diagnosed with a torn ACL in his left knee and will be out indefinitely.

A league source said Friday that Gallinari is expected to undergo surgery after the swelling in his knee dissipates and that there is no timetable for his return, but ACL tears typically sideline players for 8-12 months. Gallinari, 34, tore the ACL in the same knee in 2013.

Gallinari suffered the noncontact injury when he took a bad step during the fourth quarter of Italy’s 91-84 win over Georgia in a FIBA World Cup qualifier last Saturday. He was helped off the court and quickly ruled out of this month’s EuroBasket tournament.

Gallinari was initially diagnosed with a torn meniscus, which would require a shorter recovery time, and doctors in Italy believed that his ACL appeared stable. But a full MRI after Gallinari returned to Boston revealed the ACL tear.

In July, the Celtics signed the forward to a two-year, $13.3 million deal, and they were expecting him to provide scoring pop off the bench. Last year the 6-foot-10-inch forward averaged 11.7 points and 4.7 rebounds for the Hawks, and he presents challenges for defenses with his ability to stretch the floor with his 3-point shooting.

Now, the Celtics’ bench depth will take a significant hit. A league source said that second-year forward Sam Hauser, a 6-8 sharpshooter, will have an opportunity to grab a spot in the regular rotation with Gallinari out.

Forward Grant Williams, who emerged as a dependable defender and 3-point shooter, will likely have his role increased too.

The Celtics still have two open roster spots, and they will likely make further tweaks to account for Gallinari’s absence. They are expected to first see how things unfold at training camp, where veteran frontcourt players such as Noah Vonleh, Bruno Caboclo, Luke Kornet and Mfiondu Kabengele will battle for opportunities.

A league source said the Celtics will likely apply for a disabled player exception. If approved, they would be able to sign a player to a one-year deal worth approximately $3.2 million, or acquire one via trade. But that second option is unlikely because the Celtics still have two traded-player exceptions for larger amounts.

The Celtics are scheduled to open training camp Sept. 27, and they will open the regular season against the 76ers Oct. 18.