Danilo Gallinari, who signed with the Celtics in the offseason, is rehabbing a torn ACL. He is likely done for the season, but won’t rule out a return. Brett Davis/Associated Press

BOSTON — When Danilo Gallinari injured his knee during a FIBA World Cup qualifier, it likely ended his season. It was initially diagnosed as a torn meniscus, but further testing showed he tore his ACL, a disappointing blow after he signed with the Celtics in the offseason.

But Gallinari hasn’t ruled out returning sometime this season. Gallinari said Monday that he had surgery about three weeks before and is in the thick of rehab.

“I’m just taking it day-by-day,” Gallinari said. “That’s the best way to approach a rehab and approach an injury like this, to take it day-by-day and not really think about what’s going to happen in the future. So that’s the approach that I have.”

The rehab process will be long, but Gallinari said he’s seeing improvements on a daily basis. He added that he plans to rehab in Boston because he wants to be around the team and go to games.

It’s a tough blow for the Celtics, who hoped Gallinari would provide depth piece as a scorer and a big man. And Gallinari, 34, isn’t getting any younger. When he signed with the Celtics, he said he chose Boston because he wanted a chance to win the first ring of his career.

“The good and the bad thing is that I’ve been through this already, so I know what to expect,” Gallinari said. “But it’s always tough, especially when you approach a situation like this, coming to play for the Celtics, ready to help the team winning. It’s tough, but nothing I can do about it. We’re here and now we gotta focus on the rehab.”

The Celtics have tinkered with different big man in training camp to help fill in for Gallinari. Blake Griffin was signed while Luke Kornet is in line to carry a heavier workload. Guys like Noah Vonleh and even Mfiondu Kabengele have shown flashes during preseason games and camp as they try to make their case.

Gallinari said he knew immediately after hurting his left knee that something was wrong. He tore his ACL a decade ago, which forced him to miss the 2012-13 season. The Italian national team doctors announced it was the torn meniscus, but Gallinari said the vibes felt too similar to when he first tore his ACL.

“It’s way better to play basketball than rehabbing,” Gallinari said. “But it happens, it’s a long process. The good thing is that it’s something that the small improvements that you see every day, those are the things that make you happy and keep going and keep trying to get better every day. That’s the way it is.”

SAM HAUSER acknowledge last season how much faster professional basketball felt compared to college. He noticed while playing for the Celtics Summer League team in 2021 when he was an undrafted rookie on a two-way contract trying to earn his spot in the NBA.

Hauser feels a lot more comfortable in his role with the Celtics this season. Early in preseason, he has been impressive and Celtics fans have started to ready themselves in anticipation when Hauser rises up for a 3-pointer.

Hauser could be an intriguing piece for the Celtics this season. His shooting has improved and the Celtics are high on him to be a potential contributor and floor-spacer. With Gallinari likely out for the season, it opened the door for Hauser to have a bigger impact. To prepare, he is studying other shooters around the league, like Milwaukee’s Khris Middleton, Golden State’s Klay Thompson, Miami’s Max Strus and Duncan Robinson, and Brooklyn’s Joe Harris.

“Last year, I said it a couple of times, everything felt like it was going 1000 mph,” Hauser said. “Now, I feel like everything is slowing down a little bit and I’m getting a better understanding of playing with these guys, where I’m supposed to be, when I’m supposed to be there and things like that.”

All of the players Hauser is studying, aside from Thompson, are all relatively unheralded players who found their way in the league. Middleton and Harris were both second-round picks. Strus and Robinson were undrafted before becoming bonafide rotation players for the Heat. Of course, Strus was a Celtic and was cut before eventually making his way to Miami.

Hauser is 12 for 20 from 3-point range in the preseason and looks more comfortable with each game. Some areas are a work in progress, like defense, but Hauser is making his case for a spot in the rotation when the regular season starts Oct. 18 against Philadelphia.

“You kind of have to have the next man up mentality,” Hauser said. “Obviously, you never want to see someone go down like that, but I’ll be ready for whenever my number is called. Whatever opportunities I get, I’m excited for that.”