BOSTON (Reuters) - The Boston Red Sox suffered a setback in their battle for top spot in the American League East on Thursday when they placed third baseman Kevin Youkilis on the 15-day disabled list with a sore back.

Youkilis has played in just three games since August 10 due to the injury and the Red Sox have since slipped to one-half game behind the New York Yankees for the division lead.

"He's just been so beat up," Red Sox manager Terry Francona told Boston radio station WEEI. "Hopefully two weeks down will really allow his body to come back, and then we can have the Youk that we need for the stretch drive."

Youkilis is batting .266 with 17 homers, 30 doubles and 78 RBIs this season.

Boston, who begin an eight-game road trip Thursday against the Kansas City Royals, called up power-hitting catcher Ryan Lavarnway from Triple-A Pawtucket to take Youkilis's spot on the roster.

With regular designated hitter David Ortiz expected to miss about a week with bursitis in his right heel, Lavarnway was expected to see DH duty.

Lavarnway has 30 homers between Double-A Portland and Pawtucket this year, batting .293 overall.

(Reporting by Larry Fine in New York, Editing by Frank Pingue)