Matsuzaka looked much improved from his season debut on June 9, when he gave up four runs in five innings.
"My body right now feels a lot better than it did before I had Tommy John [surgery], and my body feels better than it did back in June," said Matsuzaka, who went back on the DL in July with a right trapezius strain.
Dice-K scattered five hits and struck out six against two walks. He threw 101 pitches and allowed one unearned run.
"When I returned back in June and I didn't get the results I wanted to, I thought for a moment that I wouldn't be able to pitch a game like today this season," Matsuzaka said. "But I got back to my rehab, my last two starts went really well and felt really good, and I knew if I had that kind of outing here, the results will come."
The results came in the form of his first victory since May 8 of last year and No. 50 for his career. Dice-K is the fourth Japanese-born pitcher to reach 50 wins, joining Hideo Nomo (123), Hiroki Kuroda (53) and Tomo Ohka (51).
Cody Ross had two hits off the Green Monster and drove in three runs. He lifted a two-run single high off the wall in the third to give the Red Sox a 3-1 lead, though he was thrown out at second. In the sixth, Ross brought Jacoby Ellsbury home with a double that nearly cleared the Monster for a homer.
"It's definitely nice to use that wall to our advantage, that's for sure," Ross said. "My swing has always been like this. I finally got to play in a place where it works."
Matsuzaka cruised through the first five innings, allowing no more than one baserunner in any frame. The Royals never seemed to be comfortable at the plate.
"What he did today and he did real effectively was he threw his slurvey slider inside on right-handers really well, really executed that pitch well a number of times," Royals manager Ned Yost said. "So when you're sitting looking out, away on him and he throws that slider that starts at you and breaks back on the corner of the plate, it keeps you off-balance really well."
Matsuzaka's only blemish came in the first, when he gave up an unearned run. Jarrod Dyson drew a leadoff walk, stole second and advanced to third on an error, then scored on a sac fly.
But Kansas City's lead was brief, as Ellsbury homered to right to tie the game at 1. It was Ellsbury's second homer of the year and his first since July 22, a span of 129 at-bats. The center fielder went 13-for-33 (.394) on the homestand, with five RBIs.
"I've seen a more aggressive hitter who is driving the ball better, hitting with some power and really aggressive on the bases," manager Bobby Valentine said. "That first run tying it up that quickly, you never know how big that is, but I think it was big."
Dice-K worked out of a jam in the sixth, the only time the Royals really threatened. Alex Gordon doubled with one out and Billy Butler walked, prompting pitching coach Randy Niemann to visit the mound. Matsuzaka induced a flyout and then struck out Mike Moustakas with a 94-mph fastball to end the threat.
"He responded, it was all him and he did a great job," Niemann said. "As he's progressed along from Tommy John [surgery], I think his fastball has gotten better, maybe not velocity-wise, but there's a life to pitches that when you're back to full strength, there's a finish and life to them that I think he's gotten back."
Royals starter Luke Hochevar was unable to replicate his previous outing, when he struck out 10 and allowed one hit over eight innings of shutout ball on Tuesday. Instead, the Red Sox knocked him around for eight hits over eight frames.
James Loney contributed an RBI single, his second in as many games with the Red Sox since being acquired from the Dodgers in Saturday's blockbuster trade.
Since that franchise-altering deal, the Red Sox are 2-1. With the outside distractions of trade speculation in the rearview mirror, Boston is focused on the task at hand.
"Now that the organization has made it's mind about which direction it wants to go, I think it's just back to baseball," said right-hander Andrew Bailey, who pitched a scoreless ninth. "With so many questions floating around the last couple weeks, now it's over with, it's done with and we're focused on the game."