Lester, 30, leaves the Red Sox at the top of his game, having gone 10-7 with a 2.52 earned run average in 21 starts. His 149 strikeouts are sixth in the American League and he is fifth in ERA.

The trade, although anticipated for several days, is still shocking in that the Red Sox are taking the unusual step of dealing a healthy and productive ace pitcher.

Jon Lester, the most accomplished lefthanded pitcher in Red Sox history, has been traded to the Oakland Athletics along with Jonny Gomes. The Sox received Yoenis Cespedes and a draft pick in return.

Lester joins an Oakland team that is in first place in the AL West. After helping lead the Red Sox to a World Series title, he has a chance to repeat that accomplishment.


The Red Sox and A’s are not scheduled to play again this season. Lester is scheduled to start Saturday for the A’s vs. the Kansas City Royals.

“Jon’s a bona fide ace who has done it on the biggest stage,” A’s shortstop Jed Lowrie told MLB.com. “That experience, it’s an intangible, having that knowledge that you’ve done it before. He’s a guy that will bolster an already good pitching staff. There’s quality arms all the way through. You look at the top four, some pretty good stuff. It’s a deep rotation.”

Cespedes, who turns 29 in October, is signed through the 2015 season. He was second in the American League Rookie of the Year voting in 2012 but has seen his batting average, on-base percentage and slugging percentage drop since.

“Cespedes is a big bat that’s going to be missed,” said Lowrie, who played for Boston from 2008 to 2011.

Cespedes has 17 home runs this season (for comparison, all Red Sox outfielders have only 14), but is hitting .256 with a .303 on-base percentage. His numbers could improve at Fenway Park, a better environment for hitters than Oakland Coliseum.


RELATED: Meet Yoenis Cespedes

Cespedes is tied with Red Sox center fielder Jackie Bradley Jr. for the MLB lead in assists by an outfielder with 12.

Gomes, a platoon outfielder, had a big impact on the 2013 World Series champion Red Sox but was hitting .234 with six home runs and 32 RBIs this season. He returns to Oakland, where he played in 2012.

As part of the deal, the Red Sox also will receive Oakland’s “competitive balance” round B draft pick in 2015. That pick will come at the end of the second round.

Lester will be a free agent after the season and said earlier this month that he would consider returning to the Red Sox. But the trade was seen throughout baseball as a sign the Red Sox were not willing to give Lester the kind of contract he was seeking and decided to generate some value by trading him now.

There was division within the organization on the matter, with many lobbying to keep Lester, saying he was worth the risk of a long-term investment.

Lester said in January he wanted to stay with the Red Sox and was willing to accept a hometown discount to make that happen. He repeated that idea at the start of spring training and expressed hope a deal could be struck before Opening Day


But the Red Sox made what now looks to be an error in judgment by offering Lester four years and $70 million, a deal not close to the market rate for pitchers of his caliber. Lester and his agents, Seth and Sam Levinson, rejected that offer in March and the sides agreed to postpone negotiations until after the season.

The Red Sox, cognizant of the kind of contract Lester was seeking, started trade discussions in recent weeks and by this week had multiple teams interested in Lester.

Red Sox players, David Ortiz and Dustin Pedroia among them, were stunned on Tuesday when it became apparent Lester would be dealt.

RELATED | On baseball: Red Sox will bounce back fine from this

“Of course you want to keep Lester around and continue playing together. He’s one of the best pitchers in the game. No question about it,” Ortiz said. “I don’t know, man. It’s frustrating to see him going somewhere else. It’s just the situation that we’re facing right now.”

Without Lester and Jake Peavy, who was traded to the San Francisco Giants last week, the last-place Red Sox will rely on a rotation headed by righthanders John Lackey and Clay Buchholz.

Young starters Rubby De La Rosa, Allen Webster, and Brandon Workman are in the rotation for the moment. They have 16 career wins.

Lester was drafted by the Red Sox in 2002 out of Bellarmine Prep in Tacoma, Wash., and found immediate success in the minor leagues. He made his major league debut in 2006 before learning that summer he had been stricken with anaplastic large cell lymphoma.


Lester was placed on disabled list in late August and underwent a series of treatments in Seattle. He was declared cancer-free in December and started the 2007 in the minor leagues. He returned to the majors in July and finished the year by winning the clinching game of the World Series.

Lester was 110-63 over parts of nine seasons with the Red Sox. He is ninth in team history in wins, fourth in starts, and fourth in strikeouts with 1,386. Only Roger Clemens, Tim Wakefield and Pedro Martinez had more.

Lester made the All-Star team three times, including this season, and was fourth in the Cy Young Award voting in 2010 when he finished 19-9 with a 3.25 ERA.

He also was durable. From 2008 to 2013, Lester made 31 or more starts each season, joining Cy Young, Roger Clemens, and Luis Tiant as the only Red Sox pitchers to accomplish that over six consecutive seasons. Lester also threw at least 200 innings five times.

Outside of the bout with cancer, Lester’s only stint on the disabled list was for a pulled lat muscle in 2011. He missed 18 days.

Lester was the team’s Opening Day starter from 2011-14 and threw a no-hitter against the Kansas City Royals on May 19, 2008 at Fenway Park. It was the first by a Red Sox lefty since Mel Parnell in 1956.

It was in the postseason where Lester made his greatest impact on the franchise. He was 6-4 with a 2.11 ERA in 12 postseason games and helped the Red Sox to championships in 2007 and 2013.


Lester was a colossus last fall, going 4-1 with a 1.56 ERA in five postseason starts. He dominated the Cardinals in the World Series, allowing one earned over 15⅓ innings in two starts and striking out 15 with one walk.

Lester won the pivotal game of the Series, beating the Cardinals in Game 5 in St. Louis to send the Red Sox back to Fenway Park with a 3-2 lead.

Lester also made an impact off the field. He partnered with the Pediatric Cancer Research Foundation to form the NVRQT (“Never Quit”) Foundation. Lester has been a frequent visitor to patients the Dana-Farber Cancer Center and a supporter of The Jimmy Fund.

RELATED | Gasper: How have Red Sox fallen so far, so fast?

In recent seasons, Lester has organized visits to the ballpark — at home and on the road — for young cancer patients and their families.

“We’re trying to raise more awareness through our foundation. Every city has a children’s hospital and a section devoted to cancer. We want to get kids out of there and to the park,” Lester said earlier this season.

There have been missteps. Lester was one of the players accused of drinking beer and eating fried chicken in the clubhouse during games in 2011 when the team collapsed and missed the playoffs. He quickly apologized, admitting it was something he should not have done.

Lester also performed poorly during the chaotic 2012 season under Bobby Valentine, going 9-14 with a 4.82 ERA. But he bounced back under manager John Farrell almost instantly the next season.

The biggest concern for the Red Sox was how Lester would fare in the later stages of his career. He has thrown 1,519⅓ innings, 17th among active pitchers in the majors. Since 2008, Lester is eighth in innings and starts.

Red Sox owner John Henry, who also owns the Boston Globe, has voiced concerns about long-term contracts for players over the age of 30, pointing out that such deals have often backfired on teams.

The Red Sox did sign Pedroia to an eight-year, $110 million extension in 2013. At an average annual value of $13.75 million, the Red Sox deemed that acceptable. Based on the current market value for pitchers, Lester will command over $20 million annually as a free agent.

Related coverage:

• Meet Yoenis Cespedes, new Red Sox outfielder

• On baseball: Red Sox will bounce back fine from this

• Christopher L. Gasper: How have Red Sox fallen so far, so fast?

Peter Abraham can be reached at pabraham@globe.com. Follow him on Twitter @peteabe.