KANSAS CITY, Mo. -- The Boston Red Sox placed third baseman Kevin Youkilis on the disabled list because of a back injury Thursday and called up catching prospect Ryan Lavarnway from Triple-A Pawtucket.

Red Sox manager Terry Francona announced the moves before the opener of a four-game series against the Kansas City Royals, which the Red Sox won, 4-3. The 24-year-old Lavarnway made his major league debut as the designated hitter, batting seventh in the Red Sox lineup. He went 0-for-4.

Youkilis, who is hitting .266 with 17 homers and 78 RBIs, has missed several games recently because of what Francona called "a back strain or sprain." Youkilis remained in Boston after Wednesday night's game against Tampa Bay to be examined by doctors at Massachusetts General Hospital. He also received an injection to alleviate some of the pain in his lower back.

"His body was so beat up anyway, I think this was the last straw, kind of put him over the edge," Francona said. "But hopefully we can get him back to where he can be Youkilis. I think after the game yesterday, it was obvious we needed to get him checked out."

The 32-year-old Youkilis was selected to his third All-Star Game this summer. He joins a rapidly growing Boston disabled list that includes designated hitter David Ortiz (bursitis in right foot), reliever Bobby Jenks (sore back), and outfielder J.D. Drew (left shoulder impingement).

"You lose one, it's kind of a big deal, you lose two it doesn't help," Francona said. "But you know, our challenge is to not let it affect us winning games, so that's what we'll do."