BOSTON -- Kevin Youkilis was a late scratch from Monday night's lineup for a second straight game because of back tightness.

"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.

Valentine takes blame for no backup plans

BOSTON -- With April coming to an end and its roughest patches behind him, manager Bobby Valentine said that he was not as well prepared as he should've been for some of the team's roster troubles.

Valentine figured Jacoby Ellsbury would be in center and that either Andrew Bailey or Mark Melancon would be closing out games, so he didn't make contingency plans. Now, Valentine has scenarios mapped out ahead of time as best he can.

"You try to have what-ifs," Valentine said Monday prior to the opening of a six-game homestand. "To tell you the truth, I didn't have a what-if at the beginning of the season that I'm kicking myself for."

Ellsbury, last year's American League Most Valuable Player Award runner-up, played just seven games before a partially dislocated right shoulder shelved him. Bailey's right thumb on his throwing hand was hurt just before Spring Training ended, while Melancon struggled mightily in his four outings. Boston traded for Marlon Byrd to play center in Ellsbury's stead, and the bullpen could evolve again very soon depending on what happens with Aaron Cook's opt-out clause at Triple-A Pawtucket.

"The outfield and the bullpen," Valentine said. "I didn't have a major plan for not having Ellsbury. My fault. I should have, and I didn't. Two-deep in the bullpen, the two guys we traded for in the winter time, I figured one of them would be pitching the ninth inning come April 13."

The Red Sox have seemed to turn a corner since they last played at Fenway Park. After seeing a nine-run lead vanish in a 15-9 loss to the Yankees on April 21, Boston went 6-1 on a road trip to push its record to 10-11.

"I think you have to have plans," Valentine said. "I'm kicking myself a little that I didn't have a great plan, but it's coming to fruition now. On the fly, the plan seems to be working."

Decision looming on Cook's future

BOSTON -- Unless the Red Sox move to a six-man rotation in brief -- "a consideration," as manager Bobby Valentine put it Monday -- Aaron Cook will not join the big league team as a starting pitcher.

Cook is 3-0 with a 1.89 ERA in five starts at Triple-A Pawtucket. If the Sox don't promote the right-hander to the big league team Tuesday, he has 24 hours to trigger the opt-out clause in his contract. If the 33-year-old does that, Boston has two options, either of which must be done within 48 hours afterward: add Cook to the 25-man roster or release him.

If Cook's going to come to Fenway, it looks like he'll have to accept a relief role.

"If he came here, yes," Valentine said when asked if Cook would be used out of the bullpen. "You don't know what's going to happen. Right now, I can't say that it can be anything other than that."

Cook made one relief appearance with the Rockies in 2011, his first relief appearance in pro ball since '03, his first full season in the Majors. Putting him in the bullpen, then, would be "challenging," Valentine said.

As to the idea of going to a six-man rotation, Valentine said, "I think that's a factor. That's a consideration, if needed."

Valentine and general manager Ben Cherington discussed Cook's situation Monday afternoon.

"We talked again on that," Valentine said. "I'm sure he has all of his ducks in order. Again, I don't know exactly when, why, how all these deadlines and all that stuff, but everyone's opinion has been shared."

If Cook's mind is made up about his own decision, Valentine wasn't giving it away. If Cook doesn't want to pitch in relief, he might want to go to another organization. Cook also could elect to stay with Pawtucket until June 1, when he has another opt-out date, but his value is high right now.

"I haven't talked to him," Valentine said. "I can't speak for Aaron."

Sweeney fitting in just fine with new ballclub

BOSTON -- When approached about the differing markets that exist in Oakland and Boston, Ryan Sweeney laughed and admitted that more pressure comes with the latter.

So far, he's thriving in it.

The left-handed-hitting Sweeney wasn't in the lineup against his former team on Monday, with southpaw Tommy Milone on the mound for the opener, but it marked his first reunion with the club he spent four years with, before joining Andrew Bailey in Boston following a deal that sent Josh Reddick to Oakland during the offseason.

Sweeney entered the day batting .373 with a .962 OPS through 18 games, along with 11 doubles. He tallied 11 doubles during the entire 2011 season.

"I know after last season, which was very difficult for him, that he refocused on what he needed to do," A's manager Bob Melvin said. "He spent the whole offseason working very hard on what I would think would be a comeback season for him. He's a terrific outfielder at every position and a line-drive hitter that probably plays very well in this ballpark. I'm glad he's off to a good start. He can slow down during this series, though."

Sweeney said he has benefited greatly from time spent in the cage with the likes of Adrian Gonzalez and David Ortiz.

"Obviously [this park] suits me a little better," said Sweeney. "I've always been the type of person when sometimes you go into a ballpark and you just feel like it's overmatched, it's so big. When you have that confidence and you go here, you know that you can hit a ball out to center or right or hit one off the Monster."