BOSTON -- The hip condition that delayed Mike Napoli's signing with the Red Sox for over 50 days is called avascular necrosis. The truth is, Napoli didn't even know anything was wrong until the Red Sox spotted the condition during a physical in December.

All along, he has felt no symptoms in his hips and has gone about a completely normal offseason.

When the Red Sox open their season on April 1 at Yankee Stadium, Napoli vows that he will be in the lineup.

After agreeing to terms with the Red Sox on a three-year, $39 million deal on Dec. 3, Napoli finally signed a contract on Tuesday, a deal that pays him a guaranteed $5 million over one season, but can max out at $13 million if he reaches his incentive clauses.

"Obviously it was a pretty tough offseason but I think we got it figured out," Napoli said. "I went through a physical and we saw some things in my hip that we had to talk about and move forward in a different direction. I think we figured that out and we're moving in the right direction. I feel like we got that behind us and we're ready to go."

According to WebMD.com, "Avascular necrosis (AVN), also called osteonecrosis, aseptic necrosis or ischemic bone necrosis, is a condition that occurs when there is loss of blood to the bone. Because bone is living tissue that requires blood, an interruption to the blood supply causes bone to die. If not stopped, this process eventually causes the bone to collapse."

In other words, Napoli's condition could eventually become an issue during his playing career. But neither side expects it will prevent the right-handed slugger from having a productive 2013 season.

"As of now, I don't have any symptoms from it," Napoli said. "I'm on medication to help me get through it. I haven't had any symptoms from it. I played with it last year, and there's no reason why I shouldn't be able to be there Opening Day and be a starter Opening Day."

"This was caught at a very early stage," said Brian Griper, Napoli's agent. "Like Mike said, he hasn't had any symptoms from this whatsoever. The first time we were made aware of it was on the physical with the Boston Red Sox. At some point, like Mike said earlier, he played with it. No symptoms whatsoever.

"Obviously he finished the year healthy, productive and, as he is right now, working out four days a week, getting ready for Spring Training, getting ready at API, hitting, throwing, getting ready for all the things for camp."

In 2013, the Red Sox expect Napoli to be a big hitter in the middle of their lineup, lending support behind Jacoby Ellsbury, Dustin Pedroia and David Ortiz.

"Well, we don't have a lot of concern about 2013," said Red Sox general manager Ben Cherington. "When it comes to health, none of us can be 100 percent in our predictions. These are human beings, and when any player is on the field, injuries can happen. We want to stay away from predictions, but there's no reason Mike Napoli won't be our primary first baseman in 2013. That's what we're counting on. There's no reason that won't happen starting Opening Day.

"I think 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, particularly by Brian and Mike, this has been caught very early. We're a long ways from Bo Jackson, and Bo Jackson's circumstance was entirely different, from what we understand. From all the information we have, there's a very good prognosis and no reason to think Mike won't be a huge part of our 2013 team."

It was a long time coming for Cherington and the Red Sox, who first targeted Napoli when the Angels placed him on waivers in August 2010. On that occasion, the Angels pulled Napoli back and the sides couldn't work out a deal by the deadline.

"I think, as everyone knows, Mike was a primary target of ours from the outset of the offseason. Mike is a hitter who has always done a lot of things that we value," Cherington said. "He sees pitches, he gets on base, hits for power, he's got a great swing for Fenway Park, a great history of performance at Fenway Park. He's also known as a terrific teammate. He's an accountable, tough player.

"And at the same time, as everyone knows, we had a desire to add offense, particularly at first base, from the outset of the offseason and we're very happy to bring Mike on board and expect him to be our primary first baseman in 2013."

Just don't look for Napoli to do any catching, barring an emergency situation. Once the hip condition presented itself, Cherington spoke with Napoli and let him know that in the best interest of everyone, it would be best if Napoli sticks to first base in 2013.

"Obviously, we know he can catch," Cherington said. "He's done that a lot in the past. He's been good at it and we would trust him back there. For a couple reasons we're focusing right now on first base. No. 1, because that's obviously our primary team need. That's where the biggest opening is. And No. 2, we do feel like it makes sense in the short term to allow Mike to focus on that position to take a bit of a load, or a bit of stress, off the body, off the lower body in particular."

The Red Sox are already deep at the catching position, with Jarrod Saltalamacchia, David Ross and Ryan Lavarnway vying for playing time.

Napoli is a powerful pull hitter who is a strong fit for Fenway Park, where he has a .307 average with nine homers, 20 RBIs and a 1.138 on-base plus slugging percentage in 75 career at-bats, including the postseason.

"I can't really put my finger on it. I've enjoyed the atmosphere [at Fenway]," Napoli said. "The fans are in the game. They know when to cheer. They really know the game of baseball. I guess it's just a fun place for me to play. Obviously, I like hitting there. Short porch to left field. I don't know. I just feel real comfortable hitting there. I like the background. I can't really put my finger on it, but it just felt good playing there."

To make room for Napoli on the 40-man roster, the Red Sox designated right-hander Chris Carpenter for assignment.

The 31-year-old Napoli hit .227 with 24 homers and 56 RBIs for the Rangers in 2012, making his first All-Star team. The Red Sox hope Napoli can get back to the level he was at in 2011, his first year with Texas, when he hit .320 with 30 homers, 75 RBIs and a 1.046 OPS.

Napoli has played 727 games in the Majors, hitting .259 with 146 homers, 380 RBIs and an .863 OPS.

Carpenter, 27, opened last season on the disabled list after undergoing surgery to remove a bone spur in his right elbow. Before joining the Red Sox in September, he posted a combined 2.08 ERA in 21 appearances between the Gulf Coast League Red Sox, Greenville, Portland and Pawtucket. With Pawtucket alone, Carpenter pitched to a 1.15 ERA with four saves in four chances in 16 games. He finished the season making eight relief appearances for Boston, and earned his one win, the first of his Major League career, on Sept. 14 at Toronto.

When the contract negotiations between Napoli and the Red Sox hit the snag over the hip condition, both sides remained confident that a deal would eventually be reached.

"The best fit for me, I think, was in Boston," Napoli said. "I had a great two years in Texas. I enjoyed my time here. But I think the fit with Boston, they stuck with me through the offseason, through this whole thing, I think it was just a good fit for me and the role I would have with the team."