“I show the scar off whenever I can,” said Green, who had open heart surgery nearly 10 months ago. “It is a part of me now. I love it. It is a reminder that I’ve battled something that a lot of people doubted I was going to come back from.”

When Green takes the court in the season opener Tuesday night, he will be defying even his own expectations. His return may be the comeback story of the 2012-13 season.

Diagnosed with an aortic root aneurysm last December and needing surgery as soon as possible, Green, 26, was among those who doubted he would ever return. When he awoke from surgery on Jan. 9, he could not move. He had a breathing tube down his throat, three drainage tubes in his chest, three IVs, a tightly bandaged rib cage, and a catheter.

He wondered: “How the hell am I going to come back and play basketball after going through this?”

The answer, he found, was that “It was like being a baby . . . I had to relearn everything.”

Over several months, Green progressed from a patient too weak to clear his own throat to a player capable of averaging 13.9 points per game and 29.3 minutes per game, as he did during the preseason earlier this month. Now, with an integral role on the Celtics assured, Green has silenced critics who thought general manager Danny Ainge foolishly signed the versatile forward to a four-year, $36-million-dollar deal in August.

“There was a maturity about how Jeff handled it from the very beginning,” said Ainge. “He was determined to come back and be better than he’s ever been. Watching Jeff and knowing him, I believed him.”

Green feels like he is “still trying to get into the swing of things” and “still learning the game and learning my teammates.” He figures he is 85 percent back, and knows he needs to work on his conditioning.

When asked how many minutes he expected Green to play this season, Celtics coach Doc Rivers said, “I don’t think we can get him enough. He needs to be on the floor a lot and we need to figure out a way of getting him as many minutes as possible.

“If we win a championship this year, it’s because Jeff Green had a lot to do with it.”

In a doctor’s office at Massachusetts General Hospital in December, talk such as that was unimaginable. When Green learned he needed heart surgery, he stayed silent for 90 minutes. He tried to comprehend the diagnosis, tried to understand how surgeons would repair his bulging aortic root with a tube-shaped Dacron graft and sew his leaking aortic valve back together. But it was too much.

“Being told you need heart surgery, everything flashes before you,” said Green. “I was young, four years in the NBA. My first thought was, ‘Everything is over with. The thing I love to do most is being taken away from me so fast.’ I just sat there with my shirt over my face and started crying.”

From there, it was a whirlwind of doctors’ appointments and tests, of talking with people who knew what he would go through. Six current or former NBA players have undergone heart surgery for aortic aneurysms during their careers, including Celtics teammate Chris Wilcox.

“We’re scar buddies,” said Green.

Aortic aneurysms in the chest are diagnosed in roughly 10,000 people each year, with the asymptomatic condition appearing more frequently in taller people. The problem often goes undetected until the aneurysm bursts. Once it bursts, there is a 40 percent likelihood of immediate death. If patients make it to the operating table after it bursts, they still have a 5 to 25 percent risk of dying during surgery.

Green was extremely lucky. His preseason physical led to the discovery of the aneurysm, which was growing at an alarming rate. Said Dr. Lars Svensson, who performed Green’s heart surgery at the Cleveland Clinic in Ohio, “One could say it was providence that we operated on Jeff as early as we did...”

By the time Green arrived in Cleveland for surgery, he had recovered from the initial shock of the diagnosis. Still, the night before the operation, he could not sleep. He played video games with his best friend, Willie Jennings.

“He was so upbeat and I wasn’t going to let him be sad,” said Jennings. He took a picture of Green holding two thumbs up before hospital staff wheeled him into the operating room.