The Boston Red Sox on Sunday traded Kevin Youkilis, a significant part of their World Series championship five years ago and a Fenway Park favorite for nine years, to the Chicago White Sox for two players, right-handed pitcher Zach Stewart and utility player Brent Lillibridge, according to team sources.

The Red Sox also will pay a large portion of Youkilis' contract.

The transaction comes days after Youkilis, 33, was told he would lose playing time to rookie Will Middlebrooks at third base and in the midst of Youkilis' worst professional season. "Youk" missed three weeks in May because of a lower-back ailment and struggled to find a stroke that once made him a critical part of a formidable Red Sox offense. He is batting .163 since June 2.

He did play – and had two hits – in Sunday's 9-4 win over the Atlanta Braves at Fenway Park. Removed from the game for a pinch-runner, Youkilis, who according to sources had grown unhappy with his diminished role, received loud applause from fans there. During the prolonged ovation, Youkilis came out of the dugout and waved. Before then, the Youkilis situation had become untenable for both the player and management.

In a season of transition for the Red Sox that has been trying for everyone, Youkilis had been among those most affected.

In mid-April, manager Bobby Valentine told reporters Youkilis appeared detached.

"I don't think he's as physically or emotionally into the game as he has been in the past for some reason," Valentine said.

Valentine apologized the following day, but the Red Sox had their first public conflict and clubhouse discord. Veteran Dustin Pedroia came to Youkilis' defense.

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"I don't know what Bobby's trying to do, but that's not the way we do things here," he said. "Maybe that stuff works in Japan."

The Red Sox were on their way to a 4-10 start, rattling Back Bay nerves and summoning memories of a September collapse that cost the club a playoff spot, then popular manager Terry Francona his job. The club has played better since, and by Sunday afternoon were 1½ games out in the wild-card race.

Once Youkilis was healthy, Middlebrooks had become one of the Red Sox's more productive hitters, creating a lineup quandary for Valentine. So, Youkilis played some third and some first, in the latter games forcing first baseman Adrian Gonzalez into right field.

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Red Sox general manager Ben Cherington shopped Youkilis for weeks, but generally found the market lukewarm. Many scouts questioned whether Youkilis could recapture the swing that once made him an All-Star and MVP candidate. He'd also won a Gold Glove at first base in 2007. There also was concern Youkilis could not withstand regular playing time at third base, given his recent history with injuries. Youkilis' salary – he's due $12 million this season, or a little more than $7 million for the rest of the season – further complicated negotiations. The Red Sox are expected to include $5.5 million in the deal with the White Sox.

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