Gagne waived the no-trade clause in his contract to make the deal official. He'll serve as Jonathan Papelbon's setup man and close on days that Papelbon is getting rest.
To land the three-time All-Star and 2003 National League Cy Young Award winner, the Red Sox packaged left-handed starter Kason Gabbard and Minor League outfielders David Murphy and Engel Beltre.
"We're excited. We feel like we have a really good club. We were looking for ways to strengthen it, and I think the bullpen is already a strength of the club," said Red Sox general manager Theo Epstein. "Acquiring a pitcher the caliber of Eric Gagne only makes us stronger. It helps give us what we hope will be a truly dominant bullpen for the remainder of the year."
Right-hander Gagne, who is expected to join the club on Wednesday, will team with Papelbon (2.15 ERA, 23 saves) and lefty setup man Hideki Okajima (0.87 ERA) as a 1-2-3 bullpen punch as potentially devastating as any in Major League Baseball.
"Eric is obviously one of the most decorated closers in recent memory," Epstein said. "He'll be joining this club primarily in the setup capacity, pitching with Hideki Okajima in front of Jonathan Papelbon and taking save opportunities here and there when Jonathan is not available. To have him agree to waive his no-trade and come here in that role means a lot to this organization."
Gagne, who was a target of the Red Sox during his free-agency period last offseason, is having a strong year (2-0, 2.16 ERA and 16 saves) for the Rangers. Because of injuries, Gagne pitched in just 16 games in a span of two seasons entering this year. However, he regained his health and his dominance in Texas.
When the Red Sox targeted Gagne back in December, the plan was that Papelbon was going to be a starting pitcher. Now, the Red Sox have two of the nastiest righty relievers in the game on one team.
"I think for us and our bullpen, it's absolutely huge," said Papelbon. "But for me, it's more special and more intriguing than I think I can really put into words. Because you've got a guy like Gagne, who is coming over here and putting his ego aside and putting everything else aside to come over here and win a World Series."
Red Sox manager Terry Francona was enthused about adding yet another weapon to his bullpen. Manny Delcarmen and Mike Timlin have also pitched well over the last few weeks, and Julian Tavarez just moved to the rotation from the bullpen, adding further depth.
"I think it's very big," Francona said. "We have a responsibility to communicate to these relievers that there's not a disappointment in what they've done. If anything, we're thrilled to death with the way they've handled their innings. We're trying to make the club better for the long haul. Our sights are pretty high. And we will communicate that with everybody, just to make sure they understand."
Gabbard, who is 4-0 with a 3.73 ERA, was scheduled to pitch for the Red Sox on Wednesday night. The homegrown product -- the Red Sox drafted him in 2000 -- had done a nice job filling in for the ailing Curt Schilling. With Schilling set to return to the rotation on either Sunday or Monday, the Red Sox would have had to take a starter out of the rotation. Now, Schilling simply takes the spot vacated by Gabbard.
"It's a good time to underscore the contribution that Kason Gabbard made this year," Epstein said. "Because at a very vulnerable time for the club, when Curt Schilling was down, he stepped up and couldn't have performed better in that role."
The left-handed-hitting Murphy was Boston's first round Draft selection in 2003. He hit .280 with 20 doubles, nine homers and 47 RBIs in 100 games at Triple-A Pawtucket this season. Beltre, who is just 17 years old, was signed as an international free agent last year. He spent this season with the rookie-level Gulf Coast League Red Sox, hitting .215 with five homers and 13 RBIs.
The most significant aspect of this deal for the Red Sox is that they didn't have to part with any of their elite prospects, a group that includes, among others, Clay Buchholz, Jacoby Ellsbury, Jon Lester and Justin Masterson.
Despite leading the American League East since April 18 -- in fact, the Sox had an eight-game cushion entering Tuesday -- they were not content to stand pat.
"I'm proud of this year's team," said Epstein. "As Tito said, we have our sights set pretty high. There's going to be a big seventh and eight inning in there somewhere where we need Hideki Okajima and Eric Gagne to get us through that, and to get the ball to Jonathan Papelbon. We had the opportunity to make this deal without dramatically impacting our farm system."
What made Gagne decide to forego his no-trade clause?
"I think it was really about the comfort of the fit, making sure that we'd be able to preserve his routine," said Epstein. "It might be in the eighth inning, more often than not, than the ninth inning. But he can still have a routine. Tito is exceptional at protecting relievers, making sure they know when they're going to pitch. I think once we were able to communicate through [agent] Scott Boras and directly to Eric that we could take care of him and make sure he stays healthy and effective, that we'd have a pretty darned good chance of doing some special things together ... it was primarily the fit."
Gagne has incentives in his contract that are bases on how many games he finishes. Epstein said the sides were able to work that out.
"His contract was set up as a closer with some incentives out there that he would have earned had he stayed in Texas in a closing role," said Epstein. "We came to a very equitable resolution fairly quickly about that."
Epstein also cited the fact that the Red Sox will receive Draft compensation for Gagne should he depart as a free agent as a factor in the deal.
The addition of Gagne seemed to be endorsed throughout the clubhouse.
"We need a guy like that to come in and do what he knows how to do," said Red Sox slugger David Ortiz. "He's got good stuff. I had about two, three at-bats against him [in June] and tried to figure him out. He really had it going on."
Epstein had someone watching Gagne pitch virtually all season.
"We've been very impressed with Eric this year," Epstein said. "Our scouts have followed him since the beginning of the season. We've been sitting on him for about the last four to six weeks. He has an excellent fastball and it's really made even better by his secondary stuff. He's got that great Bugs Bunny changeup and change-of-pace, slow curveball that will drop in for a strike, and incredible know-how. [He has] just great makeup on the mound to pitch in tight situations."
The Red Sox are hopeful that the tightest of those situations will occur late in the month of October.
Ian Browne is a reporter for MLB.com. T.R. Sullivan, a reporter for MLB.com, contributed to this report. This story was not subject to the approval of Major League Baseball or its clubs.