Andrew Bailey became a relief pitcher because he needed a mental break from starting.

That mental break has lasted three-plus years and counting — three-plus years during which Bailey has become one of the best relief pitchers in the major leagues.

Bailey was a hard-throwing 24-year-old trying to figure out how to pitch to Double-A lineups midway through the 2008 season, his second full season in professional baseball. It wasn’t working at all. He had a 6.18 ERA through 15 starts. He’d walked 15 hitters in his previous 11 2/3 innings pitched. Nothing was going right.

Oakland officials — including then-Double-A pitching coach Scott Emerson and minor-league pitching coordinator Gil Patterson — weren’t sure what to do to help him. One option involved shipping Bailey back to Single-A to get him back on track.

Instead, the Athletics put Bailey in the bullpen — just for a little while, to give him what Emerson called “a breather.”

“The plan wasn’t, ‘OK, he’s a reliever for the rest of his career,’ ” said Emerson, now the Athletics’ Triple-A pitching coach. “It was, ‘Let’s put him in the bullpen and give him a break from starting but still keep giving him his innings.’ All of a sudden, he became so dominating out of the bullpen he set the course for himself that he was going to big-league camp the next year.”

Bailey had yielded 49 earned runs through 71 1/3 innings pitched to that point. He yielded four earned runs in 39 innings pitched in 22 relief appearances the rest of the way. His strikeout-to-walk ratio climbed from 1.53 to 3.72 after his change in roles.

He was an entirely different pitcher.

“He took it as a different challenge,” Emerson said. “Some guys take that as a demotion, maybe, going from being a starter to a reliever. But, at the time, he was struggling as a starter. He was probably trying to be too pretty as a starter. This guy is a power-fastball-type guy, but, sometimes when you try to be too pretty and command the baseball too much, you leave the ball over the center of the plate.”

Bailey has never been one to shy away from a challenge.

The right-hander went down with an elbow injury at Wagner College in 2005, and had Tommy John surgery to repair elbow ligaments, a procedure that usually sidelines pitchers for a year or more.

Even after surgery, the Milwaukee Brewers made him a 16th-round pick in the June draft, but he returned to college for his senior season, taking a chance that he’d be healthy enough the following season — barely a year after a surgery — to be drafted again.

“The kid rehabbed his butt off every day — running, doing all the right things,” Wagner coach Joe Litterio said. “He was back in eight months … It wasn’t something I made him do. He did it on his own. He made sure he was back on his own. That’s the kind of work ethic he has.”

Bailey had a 2.03 ERA and struck out 53 hitters in 44 1/3 innings in his senior season.

He fell off the radar of quite a few teams after the surgery — including the Red Sox, who didn’t see him as much more than a decent prospect even before his injury.

But Oakland kept scouting Bailey through his senior year. The Athletics ensured his gamble would pay off, snatching him up in the sixth round of the June draft in 2006.

Less than a year ago, it appeared Bailey might be in trouble again. He was pitching against the Cleveland Indians in spring training when he had to leave an appearance mid-inning with elbow pain — often an ominous sign.

But the way Bailey faced the prospect of injury — with a mixture of optimism and defiance — was what set him apart.

“I was probably more upset than he was,” said Emerson, who was at that game. “When we watched him come off the field, my heart about fell to the ground … He said, ‘Hey, everything’s going to be fine. I feel pretty good.’ He’s always a positive guy, no matter what happens.”

The only time the upbeat Bailey gives way to the intense Bailey is on the mound — something that figures to serve him well in Boston.

“He understands what he’s going into,” Litterio said. “… It’s not something he’s going to back down from. He knows what he’s going into, and I know he’s excited about it.

“He has the kind of mentality where he’s the nicest guy around, but he’s a totally different person once he steps between the lines. I’ve always said that. Once he steps between the lines, he’s different –and he’s exactly what you need as a closer to go in with the focus needed to get the job done.”