With Beckett -- the ace of the staff and a 20-game winner from last season -- sidelined with back woes, Matsuzaka became the logical choice.
After witnessing the birth of his second child -- and first son -- on Saturday, Matsuzaka returned to camp on Monday and threw 58 pitches in a side session.
"I think today was a matter of him just getting reacclimated here after two days away, just getting a feel under his feet on the mound," said pitching coach John Farrell. "He threw  pitches just very low intensity."
Matsuzaka will pitch Boston's Grapefruit League finale on Wednesday against the Blue Jays in his final tuneup for Opening Day.
This won't be the first time Matsuzaka has pitched a big game at Tokyo Dome. Back on April 7, 1999, Matsuzaka made his Nippon Professional Baseball debut by pitching a gem for the Seibu Lions against the Nippon Ham Fighters.
"I love pitching in Tokyo Dome," Matsuzaka said. "I'm looking forward to that. The first game is important because it's not just for me, I have to pitch for the team. With what Opening Day represents, you have to show fighting spirit for the team."
Though Francona has known for some time that Matsuzaka would be his Opening Day pitcher, he had to make sure everything went smoothly with regards to the birth of the right-hander's son.
"Just because of Daisuke's situation, I don't want to say it was in limbo, because he was prepared to pitch," said Francona. "But there's just proper ways to do things. So he went home and things kind of broke, but he was set to go anyway. We've kind of had this in place for a while."
Clay Buchholz and Tim Wakefield will pitch the exhibition games against the Hanshin Tigers (March 22) and Yomiuri Giants (March 23), in that order.
Buchholz and Wakefield will probably pitch in Los Angeles the weekend of March 28-30, when the Sox play three exhibition games against the Dodgers.
Once regular-season action resumes with two games in Oakland on April 1-2, Matsuzaka and Lester are again likely to start.
Francona said that he did give some thought to pitching Wakefield in one of the regular-season games in Tokyo.
"The one thing we probably fought a little bit was the urge to move Wake up," Francona said. "Because if you just go on [who is] deserving and wins and losses, he's the guy that goes in there, but I think we kind of fought that urge. Just because when he's not in our rotation, we're not as good a team. We want him to make his 30 starts. If we do it in the right order where he has time to get ready, this gives him the best chance to do that."