JUPITER, Fla. -- It's commonly assumed that the St. Louis Cardinals traded for Peter Bourjos as a pure defensive upgrade.

Consider that Bourjos, from 2010-2013, has amassed more value via Ultimate Zone Rating than any center fielder but Carlos Gomez. And he's done so in fewer innings than Gomez, playing a full season in just one of the past four.

It's easy to fall into the trap of believing that Bourjos simply isn't a good enough offensive player to stay on the field, regardless of his defensive value, by looking at his games played over the past two seasons. The bigger picture is more complicated. 

Bourjos struggled in his 2010 debut at the plate, but his .271/.327/.438 slash line in 2011 was good for an OPS+ of 116. He drove the ball pretty well, with 49 extra-base hits, including 11 triples and 12 home runs. At age 24, he gave every indication of being not just a defensive specialist, but a complete player.

But the 2012 Angels made the decision to play Vernon Wells more regularly in the outfield alongside Mike Trout and Torii Hunter. Bourjos, relegated to a defensive replacement, got only 195 at-bats. And making it more difficult to get into any rhythm, those were spread out over 101 games. 

So Bourjos dedicated himself to becoming the kind of player the Angels couldn't keep out of the lineup, revamping his swing and hitting the ball on the ground more.

"Last year, coming into 2013, I worked a lot off the tee," Bourjos told me in the Cardinals' clubhouse last week. "Really concentrated, had an emphasis on staying low, going hard up the middle. Line drives, and hard ground balls, and trying to keep the ball out of the air. I made an effort to do that. 

"I set up the tee, and I had a high tee, and set up a pitch almost out of the strike zone. And I focused on hitting line drives with that, sitting on top of the baseball. And then I lowered the tee and hit the low pitch, tried to hit a low pitch through the infield. And I'd move the tee around, try a different approach, low and hard through the baseball."

And so it continued, two to three times a week at his offseason Arizona home, through November, then increasing to four, five, even six times a week straight into spring training. It was a first for Bourjos to keep hitting throughout the winter.

"It wasn't like I was taking 600 swings," Bourjos said. "But I'd say it was more like 100, 200 swings, but swings with a purpose."

That purpose, putting the ball on the ground more and achieving better results in the process, paid off for Bourjos in 2013. Bourjos posted a decent 46.8 percent ground ball rate in his strong 2011 season. By 2013, he'd increased that to 58.7 percent, good for a top-10 finish in ground ball rate among 2013 hitters with at least 170 plate appearances.

"You know it's there," Bourjos said. "It's just a matter of it coming in the game. It's a process. You get that feeling off the tee. Then you take it into BP, and then you take it into games. It doesn't come all at once. Certain things come easier. You might get that feeling in the tee, and then you get it later in the game. But eventually, it shows up.

"I felt it on and off through spring, and then April, right before I pulled my hamstring, I really felt like I was starting to get it, was consistent, felt like I had it every day. Then I pulled my hamstring. The first two weeks I came back from that, I felt great. I felt like I had really honed in on that, up the middle, a lot of line drives. When I was missing balls, they were balls in the gap, rather than what they had been, which was pop ups."

Not only did it show up for Bourjos in games, but the results were impressive enough that he'd forced his way into Mike Scioscia's starting luneup by June. On June 29, his season line stood at .333/.392/.457. His ground balls had mostly come at the expense of his fly balls, with his line drives staying pretty consistent. 

"So it was all really happening," Bourjos said. "And then I broke my wrist."

That fateful pitch came from Jordan Lyles of the Houston Astros, and obscured what Bourjos believed would be his season to prove 2011, and not 2012, better represented him as a player. 

"I really want to continue what I started last year," Bourjos said. "I really felt like I didn't get to finish that. And I think, with that approach, it's really helping me to control the strike zone. I'm starting to lay off pitches that I'd usually chase. I was hitting some pitches that I normally don't hit that well. 

"I didn't have an opportunity to play in 2012. I just sat around, I didn't do a whole lot. I came in in the ninth inning, pinch-run or come in for defense. So I never had an opportunity to show people that 2011 wasn't a fluke. And I don't think a lot of people believed I could do that. I proved a lot of people wrong. And then I didn't have a lot of opportunities in 2012 and then last year, when I was playing well, I got hurt. So it's been a tough couple of years."

He attempted to avoid surgery and rehab it, but a 5-for-46 span in August and September ended weeks before the season did, with surgery to correct it.

"It really didn't feel 100 percent until I got here, a couple of weeks into camp," Bourjos said of his wrist. "It was still sore, the strength wasn't there. But now it feels really good. It's taken some time to get back to normal, but now it feels really good."

And while opinions may differ on his offensive potential, essentially everyone on the planet reacts to Bourjos' defense the way Lucille Bluth responds to Oscar's hair.

"He hasn't been tested that much," Cardinals manager Mike Matheny said Sunday morning during a morning media availability along the first base line. "But we're watching things in between pitches. We're watching his play away from the ball. His conscientiousness -- which I talk about, catchers and center fielders, I think that's one of the main criteria for a guy who's doing exceptionally well. You take a guy who's into every pitch. He's into how we're approaching a hitter. Counts, previous swings, it's all kind of dictating what he's doing in center, and how he's approaching the other outfielders. 

"And that's all been spot-on. He's done a great job. He's always been given that reputation as a baseball rat, doing those little things. And his approach on those little details like that really support that statement."

Still, Matheny sees Bourjos as someone who can help fill out his lineup, not just support his overflowing stable of young pitchers in the field.

"He has the potential to be a very good offensive player," Matheny said. "He's got the bat speed. He's got, obviously, foot speed, which throws a whole different mindset on the defense and on the pitching. So there's things going on that he brings to the table automatically, that help him. And we think that he hasn't even tapped into the potential he has as an offensive player."