FORT MYERS, Fla. -- The Red Sox have passed the halfway point in spring training, with roughly three weeks of exhibition games behind them and roughly two more remaining prior to the April 5 season opener against the Tigers in Detroit. Yet despite the fact that the Red Sox are nearing that regular season horizon, the shape of the back end of the rotation remains, to a large degree, ill defined.

The Sox entered camp with seven pitchers competing for the last two rotation spots. However, injuries have now whittled that number down to four or five. With five turns of the rotation behind the team and three more appearances remaining for prospective starters, the need for clarity is becoming more acute.

That is true not just of the rotation but also of the bullpen, whose structure will depend in large part on those pitchers who do not open the year as starters. All of that being the case, the eagerness of manager Bobby Valentine to identify who will open the year in the rotation is becoming increasingly palpable, even though the amount of available evidence upon which to make a decision remains limited.

"It’s hard. This isn’t an exact science and I don’t proclaim to have the exact answer or the right answer in this short sampling. It’s tough," Valentine said on Tuesday night, after his team's loss to the Blue Jays. "We’ll figure it out."

The use of the future tense underscores the fact that the matter is not yet resolved. Here is a look at how the candidates for the rotation have performed to date, and the questions that face them in their attempt to open the year as starters.

Daniel Bard -- Bard made his first two Grapefruit League outings look effortless, tossing five scoreless innings, but he's had issues in his last two starts, allowing 10 runs in 7 2/3 innings and, perhaps most distressingly for Valentine, seven walks.

"I don’t think that even with his good stuff I could handle the walks," said Valentine, who noted that he is trying to figure out whether the free passes are a function of the pitcher finding his mechanics in the spring or a more unavoidable flaw.

After the right-hander issued three free passes in five innings on Tuesday, Valentine said that he was no closer to making a determination about whether the 26-year-old is better suited for the rotation or bullpen than he was before the outing. Perhaps the most notable statistical aspect of Bard's Grapefruit League performance this spring: In his 12 2/3 innings, he's walked 10 and struck out just six.

That said, with his easy delivery, Bard has still managed to show excellent velocity on his fastball, sitting in the mid-90s, and a slider that he has commanded well. His changeup also has been a promising pitch when he's used it, though Valentine lamented the fact that the right-hander largely pocketed the pitch on Tuesday.

Alfredo Aceves -- Based solely on his outings this spring, Aceves has made a compelling case for the rotation with his four-pitch mix and willingness to attack, attack, attack the strike zone. In nine innings in Grapefruit League competition, he's allowed one run, struck out eight and walked none.

He has done nothing to suggest an inability to start. The big issue remains whether the Sox will feel comfortable with the idea of taking one of the most versatile relievers in the majors out of the bullpen.

Aaron Cook -- Cook was held back at the start of spring training, and so by the time he made his first start on March 11, it seemed like he might be too far behind his peers to make his case for the rotation.

However, Cook has looked terrific so far, with a diving two-seamer and curveball that have offered glimpses of the pitcher who once was viewed as the ballast for the Rockies pitching staff. In two starts, Cook has tossed five shutout innings while allowing just one hit, walking three and striking out two.

Because the Red Sox will not need a fifth starter until April 11, if Cook remains healthy with quality stuff, his rotation chances cannot be dismissed. However, the right-hander has been dealing with some groin discomfort. While the right-hander expects to pitch through it, if that issue limits his innings buildup then it could eliminate Cook from the season-opening rotation chase.

Felix Doubront -- Doubront has shown a diverse mix of solid offerings this spring while allowing four runs in 10 2/3 innings (3.38 ERA), striking out eight and walking five. He's had some stretches in which he's looked very good -- most notably, in tossing four shutout innings against the Yankees on March 13 -- though Valentine also seemed somewhat chagrined at Doubront's recent yield of eight hits in 4 2/3 innings against the Twins on Monday.

"I didn’t see that nastiness that I wanted to. That killer pitch, that stuff that really differentiates a lot of guys. I’d like to see if he has it. I like a lot and I wished for more," Valentine said. "I don’t know if he is a favorite [for a starting spot]. I see him as a guy who’s going to pitch for our team sometime this year. I don’t know if that’s at the beginning of the season or not, but he’s the kind of guy that we’re fortunate to have. I think there’s still a step. ...

"He has a good changeup, a good curveball, a good fastball. He locates. He gets ahead, like I say, but we need that other thing that’s going to force them to put it in play weakly or strike them out. Didn’t see it today."

Vicente Padilla -- Valentine has referred to Padilla as being perhaps the team's best strike thrower, and his 9-to-1 strikeout-to-walk ratio in 10 innings (along with a 4.50 ERA) this spring represents an impressive testament to that notion. His ability to change speeds with his curveball as a complement to his low-90s fastball has been fascinating to see. 

However, Padilla has been stretched as far as just three innings this spring, and for whatever it's worth, he has yet to be the first starting pitcher in a game. Moreover, his innings buildup could be in some jeopardy after the 34-year-old suffered a mild hamstring injury on Monday; the Sox were working to make a determination about how it will affect Padilla's schedule as of Tuesday night. A setback would appear likely to all but eliminate him from consideration for the rotation. 

Andrew Miller -- The Sox have loved his stuff this spring, particularly while he has pitched out of the stretch. But Miller has been limited by a pair of injuries (elbow, hamstring) to just three innings in three Grapefruit League appearances, and so despite the fact that he has electric stuff (opponents are 0-for-9 against him with five strikeouts, and he has yet to allow a run), Valentine said he's out of the running for the rotation.

Carlos Silva -- Injured early, Silva never had a chance to compete before the Sox cut him.