"It's just back spasms. Mild," said Red Sox manager Terry Francona. "Just from going in the past, I think he knew if he stayed in the game it would cost him multiple days. I don't think we want to do that."
How did it happen?
"He just made a move out on the field and when he was running to first, he felt it and it was getting tighter and tighter and you just don't want to do something to cost him multiple games," Francona said.
Mike Lowell replaced Youkilis for the final two innings on Thursday, and went 0-for-1.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.