Red Sox manager Terry Francona said Matsuzaka is experiencing "weakness" in the throwing arm, which was not properly strengthened during Spring Training because the pitcher participated in the World Baseball Classic.
Catcher Dusty Brown was recalled from Triple-A Pawtucket to assume the roster spot of Matsuzaka, who will likely report to Boston's Spring Training complex in Fort Myers, Fla., during the team's upcoming road trip.
Francona stressed that there will be a deliberate rehabilitation program with the Japanese righty, who is 1-5 with a 8.23 ERA in eight starts this season.
"This is not going to be a two-week DL," Francona said. "We're going to have to figure this out. We have a lot of work ahead of us trying to get him back to being Daisuke. He was already with [pitching coach] John Farrell this morning looking over video, which is good.
"We'll try to do what's in his best interest, and to be honest, I think we're still trying to figure that out."
Francona cited the Classic as the primary reason for Matsuzaka's struggles, specifically diminished velocity from the two-time Most Valuable Player of the Classic.
"We never were really able to get that foundation, and he's alluded to that, too," Francona said. "We've got to try to figure it out, because it's obviously not working. He's trying to reach back and throw 93 [mph], but it's not there. As a result, he's not locating. Right now, it's been a chore.
"This is a guy that's won 33 games in two years, and now we don't have him. It's hard. I just think if we had a chance to ramp him up in our Spring Training, we'd feel a lot better."
Though he takes great pride in pitching for his country, Matsuzaka acknowledged the adverse circumstances of accelerating his preparation for the 2009 season.
"It was my fault I couldn't make that transition effectively," Matsuzaka said through an interpreter. "I felt I couldn't keep going the way I was going, so they had to make changes. I thought if I kept going like this, I was going to be a burden to this team."
"He knows he pitched before he was ready to," Francona said. "There's no getting around that. I don't know how you can. You just hope it doesn't affect your guys. He was pitching in these games trying to get outs before he was ready to do that."