A routine physical administered after Green agreed to a one-year, $9 million contract with the Celtics last week detected an aortic aneurysm.
After consulting with leading cardiac specialists, the decision was made to completely repair Green's condition, and doctors indicated to him that he should be able to resume his basketball career during the 2012-13 season.
"While we are saddened that Jeff will not be able to play this season, the most important thing is his health, and we were fortunate to have access to an amazing team of specialists to evaluate Jeff's case," Celtics president of basketball operations Danny Ainge said. "The entire Celtics family supports Jeff during this difficult time in his career."
In detailing the condition, Dr. Michael Kaplan, a senior medical correspondent for ESPN, stressed that the rupture of the aorta could be catastrophic. He said that surgery is still a major procedure, but Green, 25, indeed could resume his basketball career down the road.
"An aortic aneurysm is a condition where the abdominal aorta, which is the largest blood vessel in the body, has a dilation, meaning the wall of the blood vessel thins and then balloons out," said Kaplan, who is not involved in Green's treatment. "It's more typical in older individuals with some sort of blood vessel disease, but we see it occasionally with people as young as Jeff, usually from other conditions."
Kaplan said that the condition could be congenital, or the result of another disease process such as Marfan syndrome.
Green's contract has been voided because of the failed physical, but the Celtics retain their rights to him when he returns.
"Thank u everyone for ur thoughts and prayers," a post on Green's Twitter account read. "...much appreciated love u all..and I'll be back soon stronger and better than ever I promise."
Green will undergo surgery Jan. 9 at the Cleveland Clinic. At the request of Green, the team declined all further comments on the situation.
Kaplan said there are two types of surgery Green could undergo -- a full open-chest procedure, or a less-invasive synthetic graft replacement of the dilated segment. Either way, he cautions, it's a big deal.
"Regardless of technique, surgery is a major undertaking with long recovery and significant potential complications," he said.
Kaplan emphasized the dissimilarity between Green's condition and the one that claimed the life of Celtics star Reggie Lewis (hypertrophic cardiomyopathy).
"Hypertrophic cardiomyopathy is the most frequent reason for which we see young athletes go down and, occasionally, die on the court," Kaplan said. "That's a condition that is post-viral, where the person has a virus and the heart wall muscles are affected by it due to a thickening. It's a different situation than (what Green is facing)."
Around the league, the news of Green's looming surgery seemed to strike some players hard. Steve Nash and Baron Davis were among those sending best wishes via Twitter, and in Miami, LeBron James seemed taken aback when asked about Green after the Heat finished practice.
"I think it's unfortunate for the game to be taken away, especially after a summer like this where we already had 2½ months off," James said. "Hopefully he can get back healthy, they can figure out what's going on and he can get back on the floor. I had an opportunity to play in a few charity games with him this summer. He looked fine to me, but I'm not a doctor. I wish him the best."
James often talks about how tomorrows are not guaranteed, and the importance of enjoying each day. He also spoke Saturday of the memory of Jason Collier's death in 2005, when the center, who was with the Atlanta Hawks, died suddenly.
"As a professional, you would think every last one of us would be in the top-tier shape and nothing would be wrong with us," James said. "That's why the doctors, they are who they are."
Chris Forsberg covers the Celtics and Patriots for ESPNBoston.com. Information from The Associated Press was used in this report.