Youkilis has been bothered by discomfort in his left side, and the team is now terming the injury a left oblique strain.
To replace Youkilis on the roster, the Red Sox recalled infielder Gil Velazquez from Triple-A Pawtucket. Jeff Bailey will continue to get the majority of starts at first base until Youkilis returns.
Before the game, Youkilis tested out the injury with some on-field drills, hoping that he would feel well enough to get back into the lineup in the next day or so. That wound up not being the case.
"We're just trying to use patience," Red Sox manager Terry Francona said. "It's hard. It's not an exact science. And if we ever make a mistake, I guess we want to react on the side of caution. When it originally came out and we said day to day, I think we all really believed it. It's been more of a week-to-week thing so we'll see."
Youkilis, who is hitting .393 with six homers and 20 RBIs, will be eligible to come off the disabled list on May 20, when the Red Sox are home against the Blue Jays.
The Red Sox were also without second baseman Dustin Pedroia on Tuesday, but that was just as a precaution after the reigning American League Most Valuable Player strained his right groin in Sunday night's game. Francona said Pedroia will be back in the lineup on Wednesday and could even be used in the late innings on Tuesday if the need arises.
"He stretched and did some resistance," Francona said. "He actually, I think, reported in way better than we probably expected. He wanted to play tonight. That's probably the best sign of all. We're not going to play him tonight, but I think there's a pretty good chance he'll play tomorrow and feel good enough to not be hobbling around. That was really good news."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.