According to a Boston Globe report, Dice-K recently told the Japanese magazine Friday that he injured his right leg while preparing to help Japan defend its title at the Classic.
"Early on in January 2009, I hurt my right inner thigh," Matsuzaka told Friday, according to a Globe translator. "I consider movement around my hip joint a crucial part of my pitching motion. It happened during my exercise to strengthen my hip joint that I [have] incorporated into training since 2008.
"I may have pushed myself just a little too hard. It wasn't the pain that killed me, but it was the regrets and guilt that filled my mind. It was the time to start building up for the season, but I hurt myself because of my own doing.
"I had to make a decision whether this injury was serious enough to withdraw from the World Baseball Classic. But my body was functioning well, and by taking anti-inflammatory medicine I [could] tolerate the pain. So I continued training, but actually it was even hard just to jog."
Despite the discomfort, Matsuzaka was able to conceal the injury from the Team Japan trainers and coaches.
"I didn't want to be the center of concern for people," Matsuzaka said. "I didn't tell the trainers. Fortunately, I was in charge of my own training, so if it started to hurt, I could adjust to not hurt myself.
"But, pitching while hiding the injury was very difficult. Even when I didn't feel the pain, my body was holding back because it sensed the danger. So, my pitching motion was more of standing straight up and throwing with my upper body, relying on my shoulder strength more than usual."
Matsuzaka won each of his three starts at the World Baseball Classic, striking out 13 in 14 2/3 innings and posting a 2.45 ERA, as Japan defended its title.
"[The Classic] was hard," Matsuzaka said. "I relied on my wits and my shoulder strength. I had to be creative. I varied the paces between the pitches; I used the different kind of slider that I usually don't throw."
Matsuzaka told the magazine that he hoped the injury would heal while he continued to take a turn every fifth day in the Boston rotation, but "the condition didn't get better as the season went on."
The right-hander was roughed up for nine earned runs over just 6 1/3 innings in his first two starts of the season before being placed on the disabled list with a mild right shoulder strain. Matsuzaka returned to the rotation in late May and struggled through six more starts before going back on the DL on June 21, due to what was again called mild right shoulder strain.
"After my first stint on the DL in May, I was very hard on myself," Matsuzaka told the magazine. "Because I got plenty of rest, my shoulder was much stronger, so I could still get up there in velocity. But I couldn't use my lower body well, and I could not use my full body to generate the power.
"My fastball was not effective, therefore I lost effectiveness of my other pitches. In hindsight, it was impossible to continue faking the whole season, it was too much mental stress. But the Red Sox struggled a little bit in the beginning of the season so I wanted to help the team as much as I could."
So why did Dice-K continue to conceal the fact that he had injured his right leg in January?
"I didn't want to show my weaknesses," he said. "I didn't want them to think I was making excuses. I would rather be criticized than ridiculed for making excuses. I repeat: I really didn't want to be the center of concern for people. I believe when you say you are sick, you become sick. Sure, I appreciate that you are concerned about me, but I don't even like to be wished good luck about my health."
Matsuzaka returned in September to go 3-1 with a 2.22 ERA in his final four starts of the season.
In addition to revealing the origin of his injury problems last season during his interview with Friday, Matsuzaka also discussed meetings he had with the Red Sox to express his disapproval of the team's conditioning program.
"[That] was a very important event for me. I think I got through to [the Red Sox] that shoulder strength and pitching stamina are two different things," he said. "They have generously agreed that I can have long bullpen sessions as long as I can pass the measurement for shoulder strength.
"The reason I was able to come back strong after my second DL stint was because my thigh was healed and I was able train with the long bullpen session. It was not because I lost weight. If by losing weight you become a better pitcher, I'd lose much more. It is not that simple. I have re-started the exercise to strengthen my hip joint again."
Finally, Dice-K had a message for Red Sox fans and their fans.
"I am very sorry for making you worry," he said. "I assure you that the  season will be a great season. I am going to redeem what I lost in 2009. With my health back, I am confident and determined to produce this year. I will [try my best to] become a world champion once again."
Ed Eagle is a reporter and producer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.