The process started on Friday, when the Red Sox were awarded Wagner in a waiver claim. Ultimately, the Mets agreed to take two Minor League players to be named. But the deal wasn't completed until Tuesday afternoon's deadline, because it wasn't until then that Wagner approved it.
"Basically, he had his full no-trade clause, so he had a decision to make, whether he wanted to stay in New York with the Mets or join the Red Sox," said Boston general manager Theo Epstein. "Ultimately in the end, he woke up and today decided that he wanted to join a team that was in the middle of a pennant race and had a chance to pitch into October and had a chance to get a ring, which he's never done.
"I can't speak for the different ups and downs, the different turns the decision took for him. But in the end, he told us [that] he woke up today and he really wanted a chance to win a World Series. He's coming here for all the right reasons."
At the time the trade was completed, Wagner was in Miami with the Mets. He traveled home to Virginia and will join the Red Sox at Fenway Park in time for Thursday night's finale of a four-game series against the White Sox.
"We were looking to add another left-handed reliever and I think we added a very quality, power left-handed reliever today," said Epstein. "Obviously, his track record speaks for itself. Coming off the Tommy John [elbow ligament replacement] surgery, we understand there are limitations, but we think with the depth of our bullpen, with rosters expanding in September, that's something we can manage around and I think we stand to benefit by adding another quality pitcher."
A six-time All-Star closer, Wagner, 38, had no reservations about being one of several quality setup men the Red Sox can use in front of Jonathan Papelbon. What he wanted -- less than a year after having reconstructive elbow surgery -- was to be sure that his new team wouldn't use him in a way that would be detrimental to his health.
Major League Baseball granted the Red Sox a window of time in the hours before the deadline to speak to Wagner directly so the club could address some of his concerns.
"On his rehab, he pitched every other day a number of times," said Epstein. "We're probably not going to pitch him back-to-back days and we'll be smart about how we use him. We still think he has a lot to offer."
Wagner has an $8 million club option for 2010, but the Red Sox assured him they won't exercise it. They did, however, keep their right to offer him arbitration, which gives Boston a chance to receive compensatory Draft picks should Wagner sign with another team.
With 385 career saves, Wagner has set his sights on becoming the fifth to reach 400, then 424 -- the current record for left-handers held by John Franco, another former member of the Mets bullpen.
"I think the best deals all have present benefit and future benefit," Epstein said. "But there's no guaranteed future benefit here. There's no guaranteed present benefit. When you add a reliever, any reliever, no matter how good in the middle of a season, it's unknown what he's going to add. We scouted him, we had a number of scouts at his outings and the reports were really positive. We think he's going to help. There's no guarantee of present benefit or future benefit, but there's a chance for both."
The Red Sox will cover the remaining prorated portion of Wagner's 2009 salary of $10.5 million.
"We're excited to have him, and hopefully he can help us win a championship," said Papelbon.
Despite Wagner's gaudy credentials, the Red Sox made it clear they won't put too much on his shoulders -- nor do they need to with a bullpen that has been a significant strength all year long.
Aside from Papelbon, the Red Sox have several setup men having quality seasons, including Hideki Okajima, Manny Delcarmen, Daniel Bard and Ramon Ramirez. Okajima had been the only lefty in the bullpen.
"I think we're excited," said Red Sox manager Terry Francona. "I think we have to recognize that he's 14 months post Tommy John surgery. He's got two Major League innings. I believe five rehab innings. This is not a guy that we will lean on [heavily]. I think it's a great addition to our bullpen. But again, we will be very prudent and we'll use a lot of common sense and we'll have a lot of communication with him because of what he's been through physically."
All along, Epstein had targeted supplementing the bullpen with another lefty. To get one as accomplished as Wagner was a nice bonus.
"We're realistic," said Epstein. "He's less than a year removed from Tommy John surgery and we're not claiming that we won a playoff spot here today or anything like that. We made a move that gives us a chance to add a really quality pitcher to an already good bullpen and we're happy about that, but we want to be realistic about the expectations."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.