Jed Lowrie's two-out double to left field in the seventh inning started the letdown, as the Red Sox claimed a superb pitching duel between Lackey and his successor as the staff ace, Weaver.
"Weav's the man," Lackey said, having moved to 10-5 with relief from Daniel Bard and Jonathan Papelbon. "Weav's really come into his own and really had a tremendous year. He was great last year. He's really put it together and is one of the top pitchers in the game."
Weaver, falling to 9-7, yielded six hits and two walks while striking out eight, pushing his Major League lead in that department to 155. He was in no mood for consolation prizes, however.
"Those guys pitched with each other, pitch for pitch," Angels manager Mike Scioscia said. "We just couldn't get anything going. We shot ourselves in the foot."
Deep in the heart of Texas, meanwhile, the Rangers and Cliff Lee snuffed the A's on Nelson Cruz's walk-off homer to advance another step closer to their first postseason appearance since 1999.
Abreu was Lackey's principal nemesis in front of 40,120 at Angel Stadium, many of whom let the man on the mound in road grays know how they felt about his relocation in New England.
Abreu's eighth-inning homer, his 12th, came with one out in the eighth and ended Lackey's night.
"Lack pitched a great game," Weaver said. "I wish it would have a little different outcome. I can only do what I can do -- try to make pitches and keep us in the game. For the most part, I felt like I did that."
Abreu's two-out RBI double in the third inning had given Weaver a lead he protected until Boston rallied with two away in the seventh.
"It got away from me in the seventh," Weaver said.
Darnell McDonald's full-count walk and Marco Scutaro's single preceded Lowrie's drive that Juan Rivera was unable to glove in retreat.
"One pitch happened to get over Juan's head," Weaver said, Lowrie having taken an inside fastball the other way.
"Juan didn't quite get to a ball," Scioscia said, "but it was not an easy play."
Adrian Beltre provided a big insurance run in the eighth with his RBI double against Kevin Jepsen, who'd walked David Ortiz leading off the inning. Center fielder Torii Hunter saved a run going against the wall to take extra bases from Mike Cameron.
Lackey, departing as a free agent after eight seasons in Anaheim, was booed when he took the mound in his first appearance in a Boston uniform.
"It's definitely nice to win here, and I pitched well, for sure," Lackey said. And the booing? "That won't be forgotten, for sure."
The response was inappropriate, Weaver felt, watching from the home dugout.
"The ovation wasn't quite what I thought it would be," Weaver said. "This guy's done a lot for the organization for eight years. It's very disappointing people didn't understand. He's a great guy and was a great mentor for me. I enjoyed my time with him.
"He had to go do what he needed to do. You can't fault him for that. He got a great opportunity to pitch for a great ballclub and obviously wanted to try something new. He decided to pull the trigger for the East Coast."
For five years and $82.5 million, it was a decision that made a lot of dollars and sense for Lackey.
The big Texan sailed into the third when Abreu smacked a two-out double off the right-field wall. It scored Maicer Izturis, who'd hit into a forceout at home after Rivera's leadoff double. Alberto Callaspo's single put runners on the corners for Abreu.
Weaver got three consecutive fielder's-choice ground balls to third baseman Callaspo to escape the first inning after Lowrie's one-out double and a walk to Kevin Youkilis.
The Sox threatened again in the second on a pair of two-out singles, but Lowrie flied to center.
After Abreu's double in the third, Lackey struck out Hunter looking to leave two men in scoring position.
Weaver struck out five in the third and fourth innings but was in trouble in the fifth when McDonald -- inserted in right field when J.D. Drew developed a tight hamstring -- doubled leading off and moved up on Scutaro's fly ball. Weaver retired Lowrie on a popup and handled Youkilis on a roller to Izturis at shortstop.
The Angels hope to avoid a three-game sweep in Wednesday's matinee behind Joel Pineiro, who faces Josh Beckett.