OAKLAND - In a development few could have predicted, perhaps not even general manager Billy Beane, the A's infield has evolved into the backbone of the 2013 team.

It has outperformed what was a highly touted outfield group, and might even surpass the pitching rotation and bullpen, all things considered.

It seems like an eon ago now, but when spring training began, the A's infield situation wasn't just one big question mark, it was four. There was a measure of doubt at every position.

The double-play combination was supposed to be Scott Sizemore at second base and Japanese import Hiroyuki Nakajima at shortstop. Nakajima had never played a day in the majors and Sizemore had missed the 2012 season because of knee surgery.

There also was uneasiness about the corner infielders. Josh Donaldson had showed promise at third base, but his 2012 totals were nine homers and a .241 batting average in 75 games. Over at first, Brandon Moss hit .291 with 21 homers in 84 games, but there was uncertainty whether he could duplicate those numbers based on his journeyman resume.

When the A's couldn't re-sign Stephen Drew, who took over at shortstop for the final month and a half of 2012, questions arose about why Beane traded experienced, reliable shortstop Cliff Pennington to the Arizona Diamondbacks.

Eight months later, it all seems like so much wasted worry.

It's easy to point to the acquisition of Jed Lowrie from Houston on Feb. 4 for first baseman Chris Carter and pitcher Brad Peacock as the turning point, but at the time it appeared manager Bob Melvin just had more uncertainties to consider.

"We weren't really sure in spring training what position Lowrie was going to play," Melvin said. "It looked like Sizemore was going to be our everyday second baseman. We didn't know Eric Sogard was going to come on like he did. So we knew we might be flipping some pieces around as the season went along."

As it turned out, Lowrie almost immediately seized the shortstop position from the struggling and subsequently injured Nakajima, who spent the entire season in the minors. Sogard had a fabulous spring and forged a platoon with Sizemore. When Sizemore then reinjured the ACL in his knee during the first week of the season, Sogard absorbed an increased workload with surprising aplomb.

Then there was the stunning emergence of Donaldson, who exceeded everyone's expectations with an All-Star caliber season at third base. Even Beane was knocked back on his heels by that.

"From the time he took over last August, he showed a lot of promise," Beane said. "But he just hasn't stopped. He just kept getting better and better, and he's been an absolute grinder out there. To think how far this guy's come in a year, and to now hear chants of MVP, that's just so impressive."

Although Moss' batting average dropped this season, his production (28 homers, 81 RBIs) hasn't. In addition, Moss has improved his defense at first while maintaining the versatility to play left and right field when Josh Reddick and Yoenis Cespedes were dealing with injuries.

The infield, by comparison, has had comparatively few aches and pains. Lowrie and Donaldson both played in more than 150 games, Moss in more than 140, and Sogard nearly 130. None of them spent a day on the disabled list.

Good health is one reason Sogard thinks the A's infield was able to jell so quickly.

"Going into spring, it was kind of crazy," he said. "But once everyone finally settled in right around the start of the season, I feel like we clicked right away, and each day we continued to progress and develop a chemistry together. After that first month, it was like we were reading each other's minds and knew what we were going to do before it even happened."

Again, Lowrie was a key. Beset by injuries throughout his career, no one knew if he would hold up.

"I've finally been lucky not to have had one of those freak injuries," he said. "For years, I've always felt that was the only thing holding me back."

Added Beane, who has made no secret that he coveted Lowrie for years, "With Jed, there was never a question of whether he was a good player. It was just a matter of keeping him on the field."

While he has delivered the best offensive production the A's have had at shortstop since Miguel Tejada played there, Lowrie felt his defense suffered at times as a result of his 23 starts at second base, most of those in the first half.

"Defensively, it's been a bit of a trying year," he said. "I think I've made a lot of careless errors because it's hard to bounce around at those middle positions. Your footwork is different, arm angles are different, it's different on the double play."

The A's addressed that problem at the end of July with the acquisition of Alberto Callaspo. He hadn't played second base in nearly three years with the Los Angeles Angels and got off to a bit of a slow start, but he has come on in September to provide a strong platoon with Sogard at second and allowed Lowrie to play strictly shortstop.

Oakland also had another positive infield development with the re-emergence of Daric Barton, who was called up when Moss was forced to the outfield. Barton has responded by playing the best offensive baseball of his career, and he's adds Gold Glove quality defense at first base.

"We have had some twists and turns over the course of the season," Melvin said. "But I think the flexibility that we had allowed us to do stuff kind of on the fly, whatever was best at a particular time during the season."

Moss had a simpler explanation.

"Everybody just went out and played well and everything took care of itself," he said. "We didn't really have any revolving doors anywhere, particularly when we added Callaspo and Barton. We became a good group."

Beane agreed that the four core players — Moss, Sogard, Lowrie and Donaldson — turned a potential infield bee's nest into honey.

"We've had to mix and match here and there," Beane said. "But all in all, they've just done a great job."