Matsuzaka mentioned the ailment -- which isn't deemed to be serious by the player or the club -- in an interview with Japanese reporters on Tuesday.
"I'm not a doctor," said Epstein. "I don't want to get ahead of this thing. But if it's a mild strain like we think it is, he'll be able to have a normal Spring Training.
"He'll be delayed a bit at the start of camp. I don't want to put a number of days behind that he'll be, but he'll be a little bit behind because, while other guys are long-tossing and throwing 'pens now, he's not going to do that for a few days."
After a disappointing 2009 season in which Matsuzaka twice went on the disabled list with weakness in his right shoulder, he began a rigorous workout program in June. Matsuzaka kept the momentum going this winter by training at the Athletes Performance Institute in Tempe, Ariz.
It isn't out of the realm of possibility that Matsuzaka's back soreness was caused by the recent intensity of his workouts.
"When he was throwing on Friday, he felt a little something in his mid-upper back," said Epstein.
The team will treat the injury conservatively, which will keep him delayed at the start of camp. However, the expectation is that Matsuzaka will have plenty of time to recover from the injury and have a productive Spring Training and start the regular season in the rotation.
Communication between the Red Sox and Matsuzaka is believed to be far better than it was last year at this time, when the pitcher was training for the World Baseball Classic and spent most of Spring Training in Japan.
Matsuzaka wasn't right after returning from the Classic and later revealed that he tried to pitch through some ailments that he didn't tell the Red Sox about.
But that is in the past now, as a firmer and leaner Matsuzaka is hoping to have a big season in 2010.
"I know he's worked really hard this winter to make up for last year and come out and have a big season -- that's what we're all hoping for him," said Epstein.
All of Boston's pitchers will undergo physicals on Friday.
"We decided not to let him do much until we get the physicals, given what he went through last spring, probably doing a little too much, too soon for the [World Baseball Classic]," said Epstein. "We're two months away from when we'll need his rotation spot anyway, so we'll just slow it down, let our doctors look at him, not make something we think is really small into something big."
This will be Matsuzaka's fourth season with the Red Sox. In 2007, he went 15-12 and notched wins in Game 7 of the American League Championship Series and Game 3 of the World Series. He finished fourth in the AL Cy Young Award voting in '08, going 18-3 with a 2.90 ERA. Matsuzaka made just 12 starts for the Red Sox in '09, finishing 4-6 with a 5.76 ERA.
Matsuzaka was upfront with the club about his back problem.
"There's been really good communication," said Epstein. "I think everybody has the same goals. We don't expect there to be any issues with his programs or preparation."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.