The Red Sox right-hander's 30-day rehab assignment while recovering from Tommy John surgery was slated to end Wednesday. But a strained trapezius muscle extended the process.
The former Japanese phenom is making the best of his extended stay.
Matsuzaka yielded one hit and issued one walk over five shutout innings Saturday in Triple-A Pawtucket's 8-5 win over the Toledo Mud Hens at McCoy Stadium.
As good as the line looks, the big league veteran seemed happier with the way his neck felt after his sixth Minor League start of the season.
"I was checking how my neck would respond," Matsuzaka told The Providence Journal through a translator. "There are no problems out there, so that was a big step forward."
The 31-year-old retired the first eight Mud Hens before Argenis Diaz singled with two outs in the third. A fourth-inning walk to Matt Young was his only misstep the rest of the way.
Matsuzaka threw 56 pitches (38 for strikes), his lowest total through four starts with Pawtucket. But he acknowledged that the length of his outing had a previously determined hard cap.
"I probably could have gone more, but the original plan for today was 75 pitches or five innings, whichever came first," Matsuzaka told the newspaper. "The good thing was that I was able to pitch five innings on 56 pitches. I would have liked to continue pitching, but I'm happy with the results."
The outing was easily his best since undergoing surgery last June. He logged a 4.62 ERA in five starts for Pawtucket, Double-A Portland and Class A Advanced Salem; in two starts for the PawSox, he had allowed nine earned runs and four homers.
Because of the setbacks related to the neck injury, Matsuzaka is allowed another month -- beginning Saturday -- to rehab in the Red Sox's Minor League system. The organization can bring him back to the Majors, should it decide he's ready.
In the meantime, the former 18-game winner hopes the latest setback is nothing more than a blip on his radar.
"The ideal situation would be that my neck is fine," Matsuzaka said. "Once I come off the DL, I can make a comeback, take it day by day, see how my neck feels."
Ryan Lavarnway, MLB.com's No. 86 prospect, slugged a three-run homer -- his third of the season but first since April 17 -- as part of Pawtucket's seven-run eighth.
Tigers No. 5 prospect Casey Crosby recorded a season-high 10 strikeouts and allowed three earned runs over seven innings for Toledo.