Red Sox pitcher Daisuke Matsuzaka, right, walks off the field with team trainer Paul Lessard after being forced to leave Tuesday’s game against the Mariners.

AP / John Froschauer

SEATTLE — Unsure of what they’re dealing with beyond some “fatigue” in his right shoulder, the Red Sox are sending Daisuke Matsuzaka back to Boston for an MRI and further examination tomorrow.

Matsuzaka was removed from Tuesday night’s game before the start of the bottom of the fifth inning. After two warmup tosses, members of the training staff and manager Terry Francona visited the mound and Matsuzaka came out of the game, trailing 3-0 to the Seattle Mariners.

After Tuesday’s game, Matsuzaka said he “thought it best” to leave the outing, though he later added that he “wasn’t overly concerned” about his shoulder.

Matsuzaka will visit with Red Sox medical director Dr. Thomas Gill tomorrow morning. The Red Sox would not categorically rule out that he will miss next turn in the rotation, pointing out that because of today’s off-day, Matsuzaka could get an extra day of recovery and pitch Tuesday instead of Monday.

But it’s worth noting that the Pawtucket Red Sox rearranged their pitching plans yesterday to have Justin Masterson pitch today, which in turn would allow him to spot start for Matsuzaka on normal rest Tuesday, when the Sox host Tampa Bay at the start of their home stand.

Matsuzaka got a no-decision when the Sox came back to tie the game 3-3 before losing in the bottom of the ninth, 4-3. He is undefeated (8-0) with an ERA of 2.53, third lowest among American League starters.

In part because of language barriers, Red Sox personnel spent the better part of yesterday afternoon meeting with Matsuzaka, his interpreter, assistant trainer Mike Reinhold, pitching coach John Farrell and others members of the organization. General manager Theo Epstein took part in the meeting by phone from his Fenway office.

The first meeting lasted 90 minutes, after which Reinhold studies some strength tests, comparing Matsuzaka’s baseline measurements from spring training with tests performed more recently.

“He actually tested out pretty good,” Francona said.

But Francona later acknowledged that “putting your hand on (someone’s) shoulder is not an exact science,” leading to the decision to have him undergo the MRI.

Matsuzaka, who did not speak to reporters yesterday, joined the team on the field during batting practice and shagged fly balls in the outfield, but did no throwing. Given that he will be idle again today and tomorrow, Francona was asked if it was natural to assume that the pitcher would have to miss his next turn.

“I wouldn’t assume anything,” he said. “There’s no reason for us (to make a decision) on that now.”

Some confusion arose Tuesday night when Matsuzaka was seen grabbing his lower back as the contingent came to the mound, but Francona said that the pitcher was merely stretching his shoulder at the time, attempting to get loose. He ruled out any lower-back problem for Matsuzaka.

Francona noted that the meetings were time consuming because of language and cultural barriers faced by Matsuzaka, who speaks little English and relies on translator Masa Hoshino to communicate with club officials, coaches and teammates.

“You want to explain everything (clearly),” Francona said. “We have an obligation to explain how we feel and then listen to how he feels.”

The mere translation process, Francona said, can double the amount of time of a meeting.

The team has yet to arrive at a proper diagnosis beyond general fatigue in the shoulder area.

“I think fatigue is the proper word,” said Francona. “He was fatigued. OK, why? You want to answer those questions. If that sounds vague, I’m not really trying to be.”

As they did last year, Matsuzaka’s first with the Red Sox, the team has monitored his workouts carefully, to ensure that he doesn’t over-train and tire himself out between starts.

“We’re always trying to find a middle ground,” Francona said. “We’re very routine-oriented with pitchers; his background is not. So there’s a lot of give and take there.”

The longer schedule, workload and his own punishing workouts wore Matsuzaka down in September last season, to the point where the team had him skip a start in order to preserve his energy for the postseason.

Next Game

Tomorrow at Baltimore, 7:05 p.m.On projosports.com

For a complete report of the game, go to: projosports.com/redsox

smcadam@projo.com