Matsuzaka pitched one-plus inning, giving up four hits and five runs while walking two and striking out none and throwing a career-low 28 pitches.
It could be Matsuzaka's last start for a couple of weeks. The injury is a recurrence of the soreness the right-hander felt in his trap muscle while he was in Triple-A in late May.
Don't be surprised if Matsuzaka winds up back on the disabled list -- perhaps as early as Tuesday.
"It's the same location as [Spring Training] and two months ago," Matsuzaka said through interpreter Jeff Cutler. "I just spoke to [manager] Bobby [Valentine] about what needs to be done, and I think a decision will be made sooner than later."
While the Red Sox lost the game, they can thank reliever Clayton Mortensen for saving the bullpen.
"It's always nice to be able to go out and pick our team up. When something like that happens, it's huge to go out there and soak up some innings so our bullpen can be fresh for the rest of our series against these guys," said Mortensen. "They play us tough, so we're going to need all the pitching we can have. So it was nice, I was happy with it."
The righty came on in relief of Matsuzaka and gave the Red Sox five solid innings, giving up two hits and a run. Mark Melancon provided two scoreless innings.
"Luckily Morty came in and saved our 'pen," Valentine said. "I was scared to death once that thing started, we were going to wear out a 'pen and get a deficit going. The bad news is Dice didn't pitch very good. The good news is that Mortensen saved the bullpen."
For Matsuzaka, who is 0-3 with a 6.65 ERA, it was his fifth start back from Tommy John ligament transfer surgery.
It was the third start of Matsuzaka's career that he retired just three batters. The last one was also in Oakland, back on April 14, 2009.
While the shaky performance from Matsuzaka stood out most in this loss, the Red Sox continued to slump offensively, mustering six hits -- just one for extra bases -- against the A's.
The Red Sox have scored just 10 runs on the first five games of this road trip against sub-.500 teams, going 2-3.
"Right now our offense just isn't doing what we normally do," said shortstop Mike Aviles. "We set the bar really high for ourselves by being a good potent offense."
By the end of the night, it was hard to remember that Boston actually got off to an auspicious start. Daniel Nava opened the game with a double to right and Dustin Pedroia ripped an RBI single to right, giving Matsuzaka a 1-0 lead before he threw his first pitch.
But Dice-K gave the run right back in the bottom of the first, as Josh Reddick smashed a solo shot to right against his former team.
"My body wasn't in the condition it probably should have been," Matsuzaka said. "Despite the home run in the first inning, I got out of that inning and I thought I'd be able to fight through a few more innings."
That was not to be. In the second, Matsuzaka imploded. Seth Smith led off with a double to center and Derek Norris drew a walk. Brandon Moss, another former Red Sox outfielder, launched a three-run homer to right and the A's had a 4-1 lead.
Matsuzaka simply couldn't stop the bleeding. Brandon Inge singled to left and Cliff Pennington drew a walk, and that was all for Matsuzaka.
"I went out and asked him if he could get through it," Valentine said. "He said, 'Yeah.' Obviously it didn't look good."
Mortensen came on and walked Coco Crisp, but for the most part, the righty minimized the damage. He gave up a sacrifice fly to Reddick and then got through the rest of the inning unscathed.
"Boy, every time he's pitched, he's done well and given us those innings tonight. I had a couple of guys scheduled not to pitch in the bullpen and they might have been forced into action, and that would have really messed up the next two days. He was terrific," Valentine said of Mortensen.
The Boston bats -- stifled by Jarrod Parker and reliever Sean Doolittle -- were a different story.
"Ready to explode," Valentine said when asked about the offense. "Just ready to explode."