SEATTLE -- Boston's Daisuke Matsuzaka left his start against Seattle on Tuesday night before the bottom of the fifth inning.
Red Sox spokesman Henry Mahegan said late Tuesday that "shoulder fatigue" ended Matsuzaka's chance to join Josh Beckett as the second Boston pitcher since 1971 to begin a season 9-0.
Matsuzaka said he wasn't sure if he would be able to make his next start, Sunday or Monday at Baltimore.
"It's hard to say. We are able to assess the situation again tomorrow," Matsuzaka said through an interpreter, with inquisitors lined five deep surrounding him in an otherwise empty, quiet clubhouse.
Boston has been without starter and six-time All-Star Curt Schilling since early March because of weakness in his pitching shoulder. Josh Beckett missed the first six games of the season with a lower back strain.
"We'll certainly keep an eye on him," manager Terry Francona said. "They are going to ice it, let it cool down. ... You just always have to err on the side of caution."
The 11th Red Sox pitcher to win his first eight decisions in a season allowed three runs -- two earned -- on four hits in four innings. Then, after two seemingly routine warmup pitches before the bottom of the fifth, he appeared to briefly grab his lower back.
Catcher Jason Varitek went to the mound and manager Terry Francona soon joined them. A moment later, Francona summoned a trainer. After a brief discussion with all infielders huddled around, Francona called on David Aardsma from the bullpen. The manager and the trainer then escorted Matsuzaka back to the dugout. The Japanese pitcher then went to the clubhouse.
Matsuzaka, who struck out two, was trying to join Beckett from last season as the first Boston pitchers to begin a season 9-0 since Sonny Siebert in 1971. Roger Clemens has the best start in Red Sox history, 14-0 in 1986.
The Red Sox lost to the Mariners 4-3.
Information from The Associated Press contributed to this report.