But to get Lester, the A's part with All-Star outfielder Yoenis Cespedes, a 28-year-old who served as the big bat in the middle of Oakland's balanced offense. Minus Cespedes, the A's might be required to make another move to bolster the offense.
Lester gives Oakland a nearly unrivaled one-two punch at the top of the rotation. Beane acquired ace right-hander Jeff Samardzija from the Chicago Cubs and the rotation also includes Sonny Gray and Scott Kazmir, who could be frontline starters on other teams.
The trade will send Lester and outfielder Jonny Gomes to the A's. Lester is ticketed for free agency at the end of the season, as is Samardzija. The A's approach of building from within was tossed out of the window with two blockbuster trades in the past 26 days.
Lester, 30, is 10-7 with a 2.52 ERA, 1.12 WHIP and 149 strikeouts against 32 walks in 143 innings this season. He was scratched from his scheduled start Wednesday in anticipation of a possible trade after the two sides publicly discussed calling off contract negotiations until the offseason.
Boston is short on starting pitching, and the 2015 projected depth chart shapes up as a mystery.
Lester was having a career year in his contract season. The St. Louis Cardinals and Los Angeles Dodgers were known bidders for Lester. Lester said last week he would consider returning to Boston if he was traded away, but the Red Sox have not shown an interest in paying what is market value for their home-grown ace.
Lester reportedly turned down a four-contract worth more than $17 million per season in spring training.
The Red Sox will have Cespedes under contract for $10.5 million in 2015, the final year of his current deal. In his third major league season, Cespedes is batting .256 with 17 home runs and 67 RBI, and is a strong defensive outfielder. Plate discipline -- his on-base percentage was below .300 last season -- has been the biggest knock on Cespedes.
Gomes, 33, is hitting .234 with six home runs. He played for the Athletics in 2012, hitting .262 with 18 homers in 99 games.
The deal could signal a full-blown sell-off for the Red Sox, who traded right-handed starter Jake Peavy to the Giants last week and would be willing to move John Lackey's contract before Thursday's 4 p.m. ET deadline.
Jason Hammel, also acquired in the deal with the Cubs, is having a tough go in the American League and could be moved if the A's aren't confident in their outfield depth.
Beane's approach has almost always been anchored by the belief that pitching wins, dating to the high-profile starting rotation that included Barry Zito, Mark Mulder and Tim Hudson.