But Napoli said that because the condition was discovered in its early stages and is being treated with medication, he sees no reason why he can't play this season.
Avascular necrosis, known by its acronym AVN and also known as osteonecrosis (ON), is a progressive, degenerative disorder that kills bone tissue. According to AVNSupport.org, it is caused by a blockage or loss of blood flow to a joint or bone, causing the joint/bone to die.
"Usually, but not always, the hips are first affected, then other joints may follow," according to information on the site. "It can strike any bone or joint in the body. The bone tissue/joints actually die, just as heart muscle tissue dies from a heart attack."
Napoli has played seven years in the majors but said the condition was not discovered until he underwent a physical after agreeing to a three-year, $39 million contract with the Red Sox in early December.
The discovery of the condition caused the Red Sox to revise their original offer. Seven weeks later, Napoli signed a one-year, $5 million deal that could be worth $13 million with performance bonuses.
Napoli has two ways to boost his income with the Red Sox to $13 million this year: Stay on the active roster for at least 165 days or get 625 plate appearances with at least 120 days on the roster.
Napoli would get the full $8 million in bonuses for 165 days on the active roster, not including the disabled list.
If he doesn't achieve that, a second set of bonus opportunities kicks in: $500,000 each for 30, 60, 90 and 120 days on the active roster; $500,000 apiece for 300, 235, 350 and 375 plate appearances; and $1 million each for 400, 475, 550 and 625 plate appearances.
"Obviously, it was a pretty tough offseason," Napoli said in a conference call Tuesday, adding it was a "shock" to him to learn of the AVN. "But we think we've got it figured out. We saw a bunch of doctors, got a bunch of opinions and went from there. I got on medication. There's no reason I shouldn't be healthy and ready to go for Opening Day."
Napoli's agent, Brian Grieper, said his client is asymptomatic, noting the veteran slugger finished the 2012 season by hitting seven home runs in September, and is doing his usual preparation for the upcoming campaign. He has every expectation of fully participating in spring training and being ready for Opening Day, Grieper said.
"There is no soreness, no restrictions, no nothing," Grieper said.
Dr. Joseph Lane of New York's Hospital for Special Surgery is overseeing Napoli's treatment. Both Grieper and Napoli said doctors could not tell them what caused the condition.
"The answer to that is we don't have an answer," Grieper said. "We don't know."
Dr. Stephen Mikulak, a hip and knee specialist with California Orthopaedic Specialists in Newport Beach, Calif., said there are a host of possibilities to explain what caused the issue, including alcohol, steroid use, trauma and simple wear and tear. Of the volleyball players and bicycle racers that he's treated, for example, he attributed their condition to wear and tear. That could also be the case for a baseball catcher, as well, though the injury is an uncommon one in baseball.
Napoli said playing first base rather than catcher should take less of a toll on him physically.
According to the Social Security Disability Resource Center, if caught early, avascular necrosis can be treated through medication, rest and exercise. Later stages can require surgery, such as depression at the core of the bone, reshaping the bone, a bone transplant or even total joint replacement.
Untreated, the site said, the condition can progress to severe pain and irreversible disability in the course of just a few years.
"We don't have a lot of concern about 2013," Red Sox general manager Ben Cherington said. "There's no reason that Mike Napoli won't be our primary first baseman in 2013. There's no reason that won't happen starting Opening Day. It's very important to note that, although this condition is less common in baseball players than some other issues, from all the information that's been gathered, this has been caught very early.
"We're a long way ahead from Bo Jackson. Bo Jackson's circumstance was entirely different. From all the information we have, there's a good prognosis."
Jackson split his professional sports career between Major League Baseball's Kansas City Royals and the NFL's Oakland Raiders. He was injured after being tackled awkwardly in the 1990 playoffs and eventually had his hip replaced. He returned to baseball briefly from 1993 to '94 -- he did not play football again -- but was never the same player.
Mikulak, who has treated Olympic volleyball players and professional cyclists for AVN, said the 31-year-old Napoli has a chance of continuing his baseball career if the condition was, indeed, detected in the early stages.
"He potentially could go on and heal from this and be perfectly fine," Mikulak said.
"There's a 30 percent chance of being OK without doing anything. [If advanced], there's a 70 percent chance there will be a progressive death of bone tissue that will cause the ball portion of the ball and socket joint to collapse. And a square peg in a round hole doesn't work. He could develop arthritis and have a lot of pain. If the ball collapses, he could wind up needing hip replacement surgery [as Jackson did], or a resurfacing of the hip joints."
Mikulak qualified his remarks by saying he has not seen Napoli's X-rays or the MRI that the Red Sox administered in December that revealed the condition, and thus can't speak specifically to his case.
"Usually, but not always, the hips are first affected, then other joints may follow," according to information on the AVN website. "It can strike any bone or joint in the body. The bone tissue/joints actually die, just as heart muscle tissue dies from a heart attack."
To create roster room for Napoli, the Red Sox designated right-hander Chris Carpenter for assignment.
Carpenter was the player the Red Sox acquired last offseason from the Chicago Cubs as compensation for Theo Epstein leaving Boston to become the Cubs' president of baseball operations.
The Red Sox have 10 days to trade Carpenter before they must put him on waivers. The Cubs would be interested in bringing back Carpenter if he is placed on waivers, a source told ESPNChicago.com.
If no club claims Carpenter, the Red Sox could re-sign him to a minor-league contract.
Information from ESPNChicago.com's Bruce Levine and The Associated Press was used in this report.