He missed his fourth contest in this six-game homestand on Wednesday against Baltimore because of those spasms, a move made by manager Terry Francona after talking with Youkilis about how he felt earlier in the morning.
"[Tuesday night] we talked about being 50-50, and maybe being a little hopeful," Francona said. "But with the day off [Thursday], it doesn't make any sense, especially when he came in a little tighter than we'd hoped."
The lower back discomfort nagged Youkilis in the days prior, but it wasn't enough to keep him out of the lineup -- his flu-like symptoms were mostly to blame.
On Tuesday, Youkilis was trying to loosen up while hitting in the batting cage with about 10 minutes to go before the first pitch, and he didn't quite feel well enough to play.
Francona whipped up a second lineup card just in case, one that eventually became the team's starting lineup.
"Toward the end of BP, it wasn't getting any looser," Francona said.
Youkilis is batting .315 with 24 home runs and 94 RBIs this season.
For Wednesday's game, Mark Kotsay will get the nod at first base. It's a position he hasn't played since 2006, when he played four games at first and committed two errors while with Oakland.
"His actions are good; he just hasn't been out there very much," Francona said.
"You worry when you send a guy out there that hasn't played out there much, because you don't want them to do something they wouldn't normally do," Francona added. "But he has no problems doing this."
Mark Remme is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.