Youkilis, who played through the injury on Sunday before being scratched at the last minute from Monday's lineup, was feeling better on Tuesday. If it was up to him, he'd probably return for Wednesday afternoon's series finale against the Phillies.
However, with the team having an off-day on Thursday, Red Sox manager Terry Francona might play the conservative route with Youkilis to make sure he doesn't suffer a setback.
"He's better tonight," Francona said. "Unless he's really good, I don't know that we would do that [on Wednesday] and take chances of setting him back. We'll see. If he's OK, we could certainly play him."
Youkilis wasn't worried at all about the injury.
"It's just tight," said Youkilis. "There's no spasming right now, but that's how it was [Monday]. There wasn't spasming and then, all of a sudden, it just locked up and spasmed. I'm just going to lay low today and hopefully get in there [Wednesday]. It's just muscular. It's not like it's a disc or anything. It should be nothing big."
With Youkilis out, Sean Casey again made the start at first base and batted seventh.
The only other lineup development was center fielder Jacoby Ellsbury sitting out against lefty Jamie Moyer. Coco Crisp started in center and hit eighth.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.