Red Sox manager Terry Francona said the team had nothing official to announce. But he said an MRI exam showed no structural damage, though there was a weakness and "we're going to have to fix it. It's going to have to be addressed."
Matsuzaka's day to pitch would have been Thursday, but that's also the day John Smoltz is scheduled to make his return from rehab.
"What we decided on going forward is he's not going to make his next start," Francona said.
After a 3-0 win over Atlanta on Saturday night, Francona said that the team isn't making a move yet. The Red Sox have an off-day on Monday and Smoltz ready to pitch on Thursday.
"We're just trying to do what's right," Francona said.
Matsuzaka was 18-3 with a 2.90 ERA last season, his second in the majors after a whirlwind courtship that required the Red Sox to pay $51.11 million for the right to negotiate with the Japanese star and another $52 million for a six-year contract. But he is 1-5 with an 8.23 ERA this year after failing to get an out in the fifth inning Friday night and taking the loss.
Francona said he stayed late at the ballpark Friday night, meeting with general manager Theo Epstein and pitching coach John Farrell. Before Saturday's game, Francona spoke to Matsuzaka.
"He came in, and when we told him it became kind of obvious," Francona said. "It was a very good meeting. We needed to get him looked at."
Matsuzaka was the MVP of the World Baseball Classic this spring for the second straight time after going 3-0 with a 2.45 ERA in 14 2/3 innings over three starts. Francona said Matsuzaka wasn't the same pitcher when he returned to Red Sox spring training.
"It all started with the WBC. It's become obvious that his velocity wasn't what it was," Francona said.
Smoltz, who hasn't started since April 27, 2008, has run the clock out on his rehab and was waiting for a spot in the Red Sox rotation. The team had talked about going to a six-man staff until the All-Star break, but now Smoltz will slide into Matsuzaka's spot.
"As far as the rotation, that's not something I've ever been shook up about," Francona said. "I've never been bothered by having too much pitching."