Boston's cleanup hitter and starting third baseman, Youkilis has been playing through back woes for a while, but it finally became too much, so the club placed him on the 15-day disabled list Thursday.
In 39 at-bats since Aug. 2, it was clear that Youkilis was limited at the plate, as he hit .179 with two homers and three RBIs.
Youkilis stayed back in Boston to get an injection and didn't travel with the club to Kansas City.
"He's got a lower back strain or sprain that we kind of knew," said manager Terry Francona. "Structurally, he came out pretty sound. His body was so beat up anyway, I think this was like the last straw. It kind of put him over the edge. Hopefully we can get him back and get him back feeling like he can be Youkilis. After the game yesterday, it was kind of becoming obvious to everybody that he needed to get checked."
To replace Youkilis on the roster, the Red Sox recalled Ryan Lavarnway, one of their top power-hitting prospects, from Triple-A Pawtucket.
Lavarnway is a catcher, but he'll be used as a designated hitter during his inaugural stint with the Red Sox. He batted seventh in his Major League debut on Thursday night.
Slumping mightily at the plate as a team of late, the Red Sox will have to try to get the bats going without two cornerstones in Youkilis and Ortiz.
"Well, you lose one and it's kind of a big deal. Then you lose two, it doesn't help," said Francona. "Our challenge is to not let it affect us win a game. That's what we'll do."
Youkilis is batting .266, with 17 homers, 30 doubles and 78 RBIs this season for the Red Sox.