How long does it take to change your swing? Is it even possible to change in your 20's? The answer to these 2 questions will probably tell you all you need to know about Jackson's future with the Cubs.
Brett Jackson was drafted in the 1st round of the 2009 draft, at 31st overall. He worked his way up many prospect lists in the ensuing few years, putting up .400 wOBAs essentially in every year in the minors. Jackson displayed great plate discipline every year (he walked from 10 to 16% each season) and blossoming power (8/12/20 HR), and maintained a slugging percentage of around .490 every year.
The only problem with his game was his strikeouts, which hovered around 25% a year. The year he finally got called up, he had been striking out roughly 1/3 of the time, and he wasn't exactly tearing up the minors, either. In the majors, his strikeouts ballooned to a comical 41.5%. That's not a sustainable approach to the plate. Jackson is going to have to evolve as a hitter, most likely, to stick as an everyday player.
I've already outlined Jackson's basically stellar minor league numbers, but I'll give another quick rundown. He climbed a level in every season, and finished his AAA career at a .269/.353/.502 line. In AAA, he walked 12% of the time and struck out around 31%. I've never seen him in person (in the minors), but his admittedly small sample of major professional baseball gives us some additional insights.
First, he struck out 59 times in 142 plate appearances, "good" for 41.5% of the time. Using a sample size calculator, that 41.5% mark is anywhere from 30.8 to 52.2 as a "true" rate of strikeouts (99% certainty). Even at the extreme low end, that's a really high mark for someone of Jackson's skillset (only 14 players last year had a rate of 30% or higher, min. 250 PA).
That being said, it's relatively easy to diagnose Jackson's issues. He swings at the right pitches: he only offers at 23.9% of pitches outside the zone (league average: 29%). He swings at roughly the same percentage of in-zone pitches; however, he doesn't make contact with many of either. The league contact average is 79.7%. His contact percentage of 64.2 is the third lowest among players with 100 PA or more last year. There are two ways to be successful with a contact percentage that low: be Josh Hamilton and swing at 59% of the pitches with prodigious power, or only offer at pitches in the zone with a great amount of power. Jackson doesn't have the power to get pitchers to respect him, so he'll have to learn to make better contact. If he just became merely bad at it, his profile would rise from 4th OF to pretty good CF quickly. Hopefully his new swing helps out with that.
Jackson has always profiled as an above-average CF. He doesn't have great footspeed, but he does have good speed and he makes great jumps on the ball. He has just an average arm but throws with accuracy and will make most baserunners respect his ability to get the ball in.
I'm going to put two players side by side.
As you can see, contact in the zone (and the lack of power) is what separates Jackson from successful high-K major leaguers. Every other part of Jackson's game is major-league ready, and that includes his plate discipline. For all of Jackson's problems, he still got on base at a higher clip than Darwin Barney last year, with a wOBA greater than one Luis Valbuena. I remain very, very skeptical of the ability to retool a swing in one off-season, but if Jackson can do it (or just find some more success with his old one), he's got the ability to be a productive major-league CF for a decade to come.