"I guess he's day to day," manager Bobby Valentine said after Boston's 11-6 win over Oakland at Fenway Park. "He was in the lineup. It was during ground balls today his back tightened up again. That's what we had [Sunday] during batting practice."
Valentine said the hope is Youkilis' issue is only muscular.
"It hasn't been diagnosed yet," Valentine said.
Youkilis initially on Monday was batting cleanup against the A's. Asked about his health before the game, the 33-year-old said, "I'm in the lineup," and that he was well enough to play.
It was the fourth time in 22 games this season Youkilis has not started. The weather was cold in Chicago, where the Red Sox wrapped a four-game series with the White Sox on Sunday.
"It's probably just three days in the freezing cold," Youkilis said Sunday. "I don't know. I felt good. I felt good all week. I just woke up and it was tight. I went and did all my usual stuff and it just tightened up. Instead of missing three days or a week, they kind of took the cautious route with it."
Youkilis is hitting .219 with a pair of home runs and a .292 on-base percentage this season. The third baseman has hits in nine of his past 15 games, and he's reached safely in 11 of those 15. Often hurt, Youkilis played 120 games last season and has not reached 140 since 2008.
Nick Punto went 0-for-3 and started at third base Monday.