"I didn't think he was as good as he should be," said Red Sox manager Bobby Valentine. "His mechanics weren't very good at all, I don't think. But he's throwing pain-free, so that's good."
Matsuzaka struck out five in his fourth rehab start while recovering from Tommy John surgery. He has two starts remaining, the first on May 17 and finally on May 22, the last day he is eligible to pitch in the Minors.
"He has a couple more starts to get his whole thing together," Valentine said. "We're going to review [his mechanics] today."
Matsuzaka has allowed nine earned runs in 18 1/3 innings in the Minors, with 19 strikeouts.
"Right now he's worried about his strength and his ability to endure pitches," said Valentine, who turned 62 on Sunday.
Valentine did not rule out the possibility of going to a six-man rotation when Matsuzaka is eligible to return to the Majors later this month. The manager used a six-man rotation at times when he managed the Mets.
"It depends on who the six are," said Valentine, who also used a six-man rotation when he managed in Japan. "I guess it can work, but I don't have any plans for that right now as a long-term situation. I can see it being done a couple times through out of necessity."
But using six starters would mean a significant change in routine for Boston's rotation.
"Breaking routine is difficult for most," Valentine said. "And six days, usually you have to throw twice in between. It changes the days that they lift their weights, changes the days that they're in the training room. But if everyone is flexible enough, you never know."
Have any of the Red Sox starters expressed concern about remaining in the rotation when Matsuzaka returns?
"No one has asked me about [Matsuzaka's] situation as it relates to their situation," Valentine said. "I hope everyone in the starting staff feels that they have to compete and earn everything that they get."