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.

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)