For three years, Andrew Bailey has been one of the most dominant relievers in the majors. So why were the Red Sox able to acquire him without giving up any of their most prized prospects?

After all, the 2009 AL Rookie of the Year and two-time All-Star has a 2.07 ERA in his three big league seasons. His ERA is the sixth best in the majors and second best in the AL (behind only Mariano Rivera) among pitchers with at least 100 appearances in that time. He has struck out exactly a batter an inning, primarily on the strength of a low- to mid-90s fastball and tremendous cutter that allowed him to dominate lefties as well as righties.

According to multiple major league sources, Bailey’s career track record was one that ordinarily would have commanded a bigger prospect haul than what Oakland received from the Sox. The Athletics received a solid package of a major league-ready starting outfielder in Josh Reddick (a player whom the A’s had been trying to acquire for years), a hard-throwing lottery ticket in Raul Alcantera and an advanced hitter in first baseman Miles Head (whom the A’s will try at third base).

But under other circumstances, the A’s might have felt emboldened to dig in and demand a top prospect such as Will Middlebrooks or Anthony Ranaudo as the price for a dominant reliever with three years of team control at relatively low cost.

In this instance, however, the Sox were able to acquire Bailey in part because there are questions about his health. After logging 84 innings in 68 games as a rookie, Bailey has thrown fewer than 50 innings while making fewer than 50 appearances in each of the last two years.

That, in turn, made the acquisition cost of a trade for Bailey palatable to the Sox, especially after they reviewed the right-hander’s medicals and came away satisfied. Indeed, the fact that Bailey had an injury represented an opportunity for the Sox -- a chance to get a top arm at a slightly discounted rate.

There is precedent for excellent return on such a pitcher. Indeed, there is precedent for excellent return on Bailey in such circumstances.

In many respects, the Sox’ willingness to pursue Bailey despite some medical questions parallels the pitcher’s entry into professional ball with the A’s. In 2006, the A’s were able to grab a promising right-hander with explosive stuff in the sixth round of the draft in no small part because his injury history had kept him out of the mix of the elite college prospects in the country. Oakland was able to seize an opportunity and get an elite pitcher on the cheap.

“It’s very rare [to find an arm like Bailey’s in the sixth round]. Obviously, the circumstances lent themselves to that,” said Jeff Bittiger, the A’s scout who followed Bailey during his career at Wagner College in Staten Island. “You’ve got a little bit of a remote school so you don’t have as many eyes on him as you would, the injury, the timing of the injury where, when he comes back, it was pretty much at the end of the spring season, so not as many people took him as seriously, so a lot of people were afraid to really go out on a limb with him. But I knew from the first time I saw him throw.”

Indeed, when the A’s took Bailey, it represented the fulfillment of a two-year courtship that Bittiger had with the right-hander’s arm. The A’s Northeast area scout (who has since become a pro scout for the team) saw the powerful right-hander in a start for Wagner College, one of the smallest colleges in the country to compete in Division 1 athletics.

Expectations are typically measured for players from such small schools. Yet it immediately became apparent to Bittiger that Bailey was not the normal Northeast Conference player.

“I was just taken by, right out of the chute, the way the ball left his hand, his delivery, his competitiveness, his aggressiveness – 95 mph with cutting action, that’s tough to deal with for anyone,” recalled Bittiger. “And not only that, but he had a good breaking ball. He did jump out at me from the moment I saw the way he went about throwing the baseball. He was a standout compared to most people you see.”

Of course, Bailey had not always been able to pump that sort of high-octane gas into the strike zone, which explains why he ended up at Wagner (rather than a baseball powerhouse) in the first place. Wagner coach Joe Litterio suggests that Bailey had been a big kid who threw in the mid-80s and had some command difficulties coming out of high school.

There had been glimpses of excellence during Bailey’s freshman year (Litterio recalled a 10-strikeout game against Monmouth), but he struggled to a 2-7 record and 6.79 ERA that year. It wasn’t until his sophomore year that Bailey returned to campus as a legitimate prospect.

Even in the controlled environment of throwing indoors to teammates in a batting cage prior to the start of the season, Bailey’s fastball started hitting 90 in the spring of his sophomore year.

“You could see his sophomore year that he knew he had something,” said Litterio. “When he came back his sophomore year, he was a different kid. His attitude changed, his demeanor changed, his confidence level changed. He went out and after that summer and dominated on the mound.”

Bailey went 6-4 with a 3.18 ERA in his sophomore season along with 84 strikeouts in 76 1/3 innings. By the time he returned for his junior year, with more velocity on his fastball and a curveball that had the potential to be a swing-and-miss pitch, it was clear that he had a professional future.

That fastball graded for Bittiger as a clearly above-average pitch (a 60 on the 20-80 scouting scale) with the potential to be well above-average (projected as a 70). There was, however, effort in his delivery, and his attacking style (“He wanted to challenge people more than making pitches in the strike zone,” noted Bittiger) suggested to the scout that the bullpen might be in his future.

“I thought his personality and demeanor might lend itself better to a short reliever,” said Bittiger. “Some guys can pitch on adrenaline and are best suited for short relief. I thought with his bulldog mentality, that could be a thing for him in the future.”

However, the future would have to wait. Bailey got off to an excellent start in his junior year, going 3-2 with a 3.46 ERA, 47 strikeouts and 12 walks in 41 2/3 innings, but blew out his elbow after seven starts. He required season-ending Tommy John surgery.

Even so, Bittiger remained convinced that Bailey could rehab and come back as an impact arm. In the A’s draft room in 2005, he agitated for Oakland to take the right-hander, and was crestfallen when the Brewers swooped in and tabbed him in the 16th round (No. 475 overall).

“We were about to do it,” said Bittiger. “Thank goodness they couldn’t come to terms with him. I thought I’d lost him to Milwaukee.”

(Random aside: The 16th round of the 2005 draft proved a prospect-studded one. The Brewers drafted but did not sign Bailey; the Orioles took hard-throwing right-hander David Hernandez; the A’s, having missed the chance to nab Bailey, selected Justin Smoak (whom they failed to sign); and the Twins took Yonder Alonso, who did not sign, went to Miami, and recently became the key prospect of a trade sending Mat Latos from the Padres to the Reds.)

Bailey thought he was better than a 16th-round pick, and was intent on proving it. While the typical trajectory of recovery from Tommy John surgery is 12-14 months, Bailey, in the words of A’s scouting director Eric Kubota, was “well inside that window” when he returned to the mound late the following spring.

It was a gutsy move by Bailey to stay in school. From a career standpoint, the simplest path for Bailey would have been to sign with the Brewers, rehab under the supervision of a dedicated baseball training staff and return at a measured pace.

Instead, he rolled the dice with his career, committed himself to a blistering rehab pace and made his way back to the mound in time to make 12 appearances (six starts) in 2006.

“That’s a risk, when you turn [a pro career] down to come back to a college season that is not very long. He had to come back in eight months from rehab from Tommy John just to be able to pitch for us. He worked his tail off to do that,” said Litterio. “Some guys don’t come back from Tommy John. It’s very risky and shows the type of kid he is. He rehabbed the heck out of it.”

He hadn’t been a terrifically prominent prospect before the surgery, and so when he returned to the mound in the later stages of the following season, some scouts were off his bandwagon given that there would be relatively few opportunities to see him, particularly in conference play.

But Bittiger stuck with him and was rewarded for doing so.

“I got to see him late in the spring. I was one of the few guys there that day,” said Bittiger. “Watching him warm up, it was very impressive. I couldn’t see any evidence of him altering anything delivery-wise because of the injury. He came out and I think touched 96 that day. The breaking ball was a little bit behind, but the breaking ball is usually later coming back.

“But I’m saying to myself, ‘If he’s throwing 96 now, 12 months removed from surgery, I’m back on board with this guy and I’m going to push hard because I really want him.’”

Bittiger was pulling at Kubota’s sleeve as early as the third round in the Oakland draft room in 2006, and the A’s ultimately pulled the trigger in the sixth round. Bailey was signed for $135,000. Once in Oakland’s system, the A’s found a pitcher who, according to Kubota, “pretty much came as advertised. He became kind of what we thought we were getting.”

Though Bittiger had suggested that relief might ultimately prove his forte, Oakland thought that he had a chance to start and he showed promise in the rotation until he hit a wall in Double-A. After struggling badly with Midland in 2008, he was moved to the bullpen, where he dominated.

“That helped him in his career – instead of trying to maintain for six innings, seven innings, he could go out and say, ‘OK, I can let it go.’ I think that’s what kind of turned him around,” said Litterio. “The idea that he can go out, throw as hard as he wants and not have to worry about the third or fourth time through benefited him.”

Bailey proved the proverbial fish to water in relief, something that positioned him as one of the dominating relievers in the game and that ultimately positioned him to be traded to the Sox. The A’s are focused on contending a few years down the road; in the interim, three years of a closer for a team destined to post losing records has far less value than players who might have a chance to be regulars beyond that timetable.

For the Sox, Bailey represented an elite arm, one whose early-career success and sometimes-overpowering stuff make him a natural heir to Jonathan Papelbon as the team’s closer in 2012 and beyond.

Still, there are -- at least in theory -- questions.

Bailey has spent his career closing in relative obscurity for the A’s. Because Oakland has been out of contention during the right-hander’s career, he has yet to perform under the most glaring spotlight.

Yet the new environment will not be foreign to Bailey, who described the feeling of running to the mound from the bullpen at Fenway as one of his career highlights.

“He’s the type of kid who, when he gets on the mound, everything that’s happening doesn’t matter. It stops for him,” said Litterio. “[Questions about how he will handle Boston] make me laugh, because he’s a Jersey kid. He grew up in this area – the New York pressure, the Boston pressure, he’s grown up around it.”

And then, of course, there is the matter of health. Bailey, in his introductory conference call with the media following the trade, said that this has been the healthiest offseason of his big league career, the first time that he hasn’t been rehabbing from a surgery.

His injury history does raise some questions about his long-term ability to stay on the mound. Yet if history is a guide, that fact is as much an opportunity for the Sox as a source of caution.

“When he came back from Tommy John in eight months, he dedicated the time to stay healthy,” said Litterio. “This is his dream. I know that he’s going to take full advantage of this and do whatever it takes to stay healthy.”