All spring, it seemed like the rotation was the unsettled part of the Red Sox. After all, the team spent the entire exhibition slate with a TBA slapped upon the last two starting spots, with the performances of as many as six starting candidates being treated like the shifting fates of the Dow Jones.

But on the day when the Sox defined the shape of their rotation, pegging Felix Doubront and Daniel Bard for the fourth and fifth spots, the bullpen seemingly drifted into a state of precarious uncertainty. That sense was the byproduct of a surprise injury that may force the Sox to redefine the penciled-in responsibilities of their relievers at the start of the year. 

Manager Bobby Valentine admitted at times during the spring that he was perplexed by his bullpen, feeling that it was difficult to give it definition since its contributing members -- some of whom remained in competition for starting jobs -- were unknown. Aside from closer Andrew Bailey, Valentine said, he did not yet have a blueprint for the late innings.

That admission, in turn, imparted particular significance to the disclosure by Valentine and the Sox on Sunday that Bailey is dealing with what the manager deemed a “right thumb situation.” The 27-year-old will be examined in Boston on Monday as the team tries to discern both the nature and severity of the injury. A major league source acknowledged that a season-opening stint on the 15-day disabled list is a possibility.

Given that teams build their bullpens from the closer backward through the middle innings, with all other relievers settling into roles around the man responsible for the final three outs of a save situation, it seemed fair to wonder whether the Sox are in trouble for the late innings.

After all, the Sox bullpen struggled down the stretch last year, despite the presence of closer Jonathan Papelbon and setup man Daniel Bard. With Papelbon in Philadelphia and Bard moving to the rotation, there had been suspicion that the Sox were courting instability in the late innings, even after trading for both Andrew Bailey and Mark Melancon in the offseason. With Bailey potentially unavailable at the start of the season, the sense of uncertainty is only magnified.

Still, the idea that Bailey could be sidelined for the start of the season may not yet be worthy of alarm bells. If the injury is deemed minor, then the Sox have the luxury of backdating the closer’s DL stint -- if he is indeed to go on the disabled list at all -- to March 26 (the day after his last Grapefruit League game), thus allowing them to activate the right-hander by April 10. In such a scenario, Bailey would miss just four games to start the year.

Moreover, the idea of Bailey missing some time is not an unexpected development. When the Sox traded for the right-hander, they were hopeful but realistic about the workload he would handle.

With knowledge of an injury history that has included two surgeries and two lengthy DL stints in his three-year major league career, the Sox fully expected that they would have to measure Bailey’s innings in order to give him the best chance of being healthy for the longest possible period of time. The team recognized all along that closing contingency plans would be needed.

That was part of the reason why the acquisition of Melancon -- who spent last season closing for the Astros -- was a significant complement to the deal for Bailey. The Sox already had a built-in fallback plan for those times when Bailey’s workload would be regulated either by design (the team wanting to give him rest) or necessity (injury).

Hence, while this particular injury came upon the Sox as a surprise, the team is not caught with its guard completely down.

“I think things have been done accordingly to make sure we're covered there,” Valentine told reporters in Fort Myers on Sunday. “With [Bailey], there has to be a plan if he can't go and now it's that plan plus somebody else, but I don't think it's going to be somebody else on a daily basis.”

That statement suggests that the Sox needed to have a fallback plan for days when Bailey was unavailable; on such days the Sox would require not only a fill-in closer but also a fill-in setup man to help bridge the game to Bailey’s surrogate.

The Sox now would appear to have those two spots covered. Melancon represents one option for the late innings, and Alfredo Aceves -- who was informed on Sunday, to his chagrin, that he will be back in the bullpen rather than in the rotation to start the year -- represents another.

The combination of Melancon and Aceves (as well as the promise shown in relief by Vicente Padilla, who has 11 strikeouts and just one walk in 12 2/3 innings of Grapefruit League play) means that the short-term absence of Bailey, while less than ideal for the Sox, is not likely destabilizing.

A longer-term injury would be another matter entirely, as the erosion of depth tends to manifest itself over weeks rather than days. For now, however, the Sox think they can manage for a time without Bailey.

“I have faith,” Valentine told reporters. “This has all been happening quickly, we've been having a lot of meetings today and I haven't formulated everything in my mind and I don't know that I have to, but it seems that it's OK, it seems to me that with Aceves being a very key component three days ago, he's even more of a key component now.”

All the same, even if he misses no more than a handful of games, the Bailey injury will raise at least a yellow flag. After all, Papelbon never went on the disabled list as a member of the Red Sox. His lone injury occurred in September 2006, when he suffered a shoulder subluxation.

After that, the Sox closer developed a religious commitment to his strength and conditioning program that made him something of a freak. He was one of just four pitchers to appear in at least 55 games in each of the last six years. There were some days when he was unavailable due to fatigue or workload issues, but almost never a physical injury that kept him from getting on the mound.

Bailey does not have that track record. Indeed, if he lands on the disabled list, it would mark the third straight season in which he has been on the D.L. In the previous two years, the right-hander has been sidelined by an intercostal strain (July 2010), had surgery on his right elbow (September 2010) and suffered a strained right forearm (March 2011). He also required surgery after the 2009 season on his knee, and was held back at the start of this year's exhibition season after suffering a mild lat strain during a test of his vertical jump. 

In other words, Bailey (2.07 career ERA, 9.0 strikeouts per nine innings) has been nearly as dominant a reliever as Papelbon (2.33 ERA, 10.7 strikeouts per nine innings) to this point in his career, but due to injuries he has been far less capable of staying on the mound than his Red Sox predecessor. While the current thumb injury may be no more than a blip in his season, it is nonetheless a reminder that there is one discernible distinction already evident when comparing the new Sox closer and the old one.

THE FALLOUT

Red Sox players potentially impacted by the injury to closer Andrew Bailey:

Alfredo Aceves -- Valentine said that the decision to keep Aceves in the bullpen rather than moving him to the rotation was driven in part by the injury to Bailey. Valentine described Aceves as a pitcher whom he could envision using at the end of a game, suggesting that the 29-year-old will be considered as a possible fill-in closer.

Mark Melancon/Vicente Padilla -- Each would appear likely to move up the bullpen pecking order while Bailey is down. Like Aceves, both could emerge as an option to close.

Scott Atchison/Justin Thomas/Junichi Tazawa/Clayton Mortensen -- The injury to Bailey could result in the Sox adding an extra pitcher to their bullpen to start the year. The leading candidates who remain in big league camp would appear to be right-hander Atchison and lefty Thomas. Atchison has turned in his typically strong spring, with a 2.84 ERA, nine strikeouts and two walks in 12 2/3 innings. Thomas, meanwhile, could be added as a second lefty to join Franklin Morales in the bullpen. In 10 Grapefruit League games, the 28-year-old has a 4.50 ERA, eight strikeouts and three walks. Notably, if added to the 40-man roster, Thomas could be optioned to the minors without exposing him to waivers. The same isn't true of Atchison, who is now out of minor league options.