But once he exited after six fine innings -- two hits, one run, four walks, four strikeouts -- it all fell apart. A collective bullpen meltdown in the top of the seventh inning led to the disappearance of a two-run lead and, ultimately, a 7-5 loss to the Angels in the rubber match of the three-game series.
"Unbelievable," said Sox reliever Manny Delcarmen. "Normally, a guy gives us six innings and we go right to our bullpen and shut the door and [Jonathan Papelbon] comes in for the save and game over. It didn't happen today, but everyone is real happy with what he did. I'm pretty sure he'll be back up and do it again and we'll pick him up next time."
Masterson was sent back to Double-A Portland after the game and right-hander Bryan Corey -- who was designated for assignment on April 17 before clearing waivers and going to Pawtucket -- will re-join the Red Sox on Friday, taking Masterson's spot on the roster.
What pleased Masterson most about his performance?
"Really, just putting the team in an opportunity to win," said Masterson. "That's what the starting pitcher's role is to do. Coming up, I was hoping to help out. There are some unfortunate things that happened and we didn't get the victory today, but I just wanted to go in and throw a lot of strikes."
After the game, the Sox packed their bags and headed to St. Petersburg, Fla., for a three-game series against the Rays. During a seven-game homestand in which several players were sidelined by the flu while others remained idled by injures, Boston managed to go 5-2.
Masterson, known for his sinkerball, got 11 of his 18 outs on the ground. He threw 95 pitches.
"He handled himself just like you would hope," said Red Sox manager Terry Francona. "That's a lot to ask a kid coming out of Double-A. He competed, he had composure, he knew what he wanted to do and he did it. He did a great job. As an organization, I don't think we could be prouder."
Immediately after Masterson left, the bullpen got into trouble. Javier Lopez opened what turned out to be a four-run seventh, giving up a walk and a hit.
"Leadoff walks are always tough to get around, especially in the late innings," said Lopez. "It burned us today. It was tough, because the guy made his debut and gave us six great innings, we couldn't close the deal for him."
On came Delcarmen. The right-hander, who had been sidelined with the flu the last two days, walked Mike Napoli. Then Delcarmen got a grounder to the right side that first baseman Kevin Youkilis tried to make a play on, but it wound up as an RBI infield single to bring the Angels within one.
"I can't make an excuse," said Delcarmen. "I threw four pitches like nowhere near the zone. I didn't feel great. I tried to go out there and get the job done and it just didn't happen today."
With the bases still loaded with nobody out, Francona looked for Hideki Okajima to bail the team out of a sticky situation. But Okajima was greeted with a game-tying RBI single from Chone Figgins and a go-ahead, two-run single by Gary Matthews Jr.
The Angels bumped their lead to 7-3 with a run in the eighth against David Aardsma and another on the ninth off of Julian Tavarez.
Down to their last out against Angels closer Francisco Rodriguez, David Ortiz cranked a two-run shot to right field -- his second in as many nights -- to bring the Sox within two. Manny Ramirez followed with a deep blast to right-center, but it was flagged down on the track to end the game.
Ortiz's revived bat was one of the best aspects of the homestand. He has hit safely in 10 of his last 11 games with three homers, two doubles and 17 RBIs.
"He's starting to look dangerous -- that's good to see," said Francona.
The Red Sox had a chance to blow this one open early, loading the bases with nobody out in the second. But Julio Lugo and Kevin Cash both stuck out. In between, Coco Crisp got a run home on a fielder's-choice grounder to make it 1-0.
Masterson kept it right there until the Sox extended their lead in the bottom of the fourth. With one on and two outs, Crisp drilled a ground-rule double to right-center to bring home Youkilis. Cash followed with another ground-rule double, making it 3-0.
The Angels got one back in the fifth, when Napoli hammered a solo homer to left. Other than that, the Angels accomplished next to nothing against Masterson.
While the Red Sox went southward to Florida, Masterson headed back north to Maine.
"I expected that was going to happen," Masterson said. "Come up and, hopefully, do well and continue to show them that I'm ready to go. When the time comes that I'm needed, that's when I'll be up here."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.