Dice-K is headed to the DL with an elbow injury.
Francona wasn't sure how long the Sox would be without Matsuzaka.
"It's impossible," Francona said Tuesday, when the team's game against the Baltimore Orioles was postponed because of rain. "If you put a time period out there, that's not fair to Dice-K. The doctors don't even know."
The Red Sox made the announcement about the rainout shortly before Tuesday's game was scheduled to start as rain created puddles on the tarp covering the infield. Temperatures were in the high 40s and dropping.
No makeup date was immediately announced.
Matsuzaka will be the second Boston starter on the DL this week. The Sox also placed right-hander John Lackey on the 15-day DL on Monday with a strained right elbow, leaving them without a chunk of their rotation indefinitely.
"We're going to get Lack healthy and get him back out there as quickly as we can," Francona said. "We're going to do the same thing with Dice-K."
Matsuzaka had an MRI on Tuesday. Francona says the elbow is sprained and Matsuzaka will be replaced on the roster by RHP Michael Bowden, who was being recalled from Triple-A Pawtucket.
The Red Sox had not yet officially made the move as of Tuesday night, when their home game against Baltimore was postponed because of rain.
Knuckleballer Tim Wakefield was scheduled to start Tuesday, but will probably go Sunday instead, Francona said. RHP Clay Buchholz was scheduled to start Wednesday when the Sox open a home two-game series against Detroit — weather permitting.
Matsuzaka also left a start late last month against Seattle with tightness in his elbow. Matsuzaka started Monday's game against Baltimore, but had control problems as he walked seven in 4 1-3 innings.
The Red Sox had the MRI done Tuesday as a precaution.
"I know he's had command issues in the past," Francona said. "That's always something to think about when people aren't commanding. It doesn't have to be just a decrease in velocity."
Francona did have some good news about his beleaguered staff: Reliever Bobby Jenks threw from 60 feet and had good velocity. Francona said Jenks will move back gradually through the week before working his way back to the mound. Jenks went on the DL on May 5 with a strained right biceps.
"He did very well. That was good," Francona said.