The 2017 season has been defined by breakout seasons from veteran sluggers: Eric Thames returned from Korea and seemingly hit a home run every night. Ryan Zimmerman followed the advice of Daniel Murphy and ended April in pole position for the National League Triple Crown. Bryce Harper is back to his NL MVP Award-caliber performance of 2015 (Ready to feel old? Only 24, Harper is already in his sixth big league season).
There's one player whose breakout is a bit less expected: Yonder Alonso, who homered against the Twins in the A's 9-1 loss on Tuesday night.
Since being selected seventh overall in the 2008 Draft, the 6-foot-1, 230-pound lefty swinger has been the rarest of anomalies: a slap-hitting first baseman. Alonso's career slugging percentage entering this year was .387 -- the lowest number for any active first baseman with at least 500 career plate appearances -- and he's yet to reach double-digit homers in a season.
That's all in the past. Like many players, Alonso has been reborn with the fly ball. With players like Murphy attributing their breakout to the glory of loft -- and the advent of Statcast™showing the benefits of getting the ball in the air -- Alonso has drastically changed his swing.
In 2016, Alonso's average launch angle was just 10.3 -- a few ticks below the league average of 10.8. It's not a death knell for production, as players like Corey Seager and William Myers had similar numbers, but Alonso's swing was geared for ground balls. This year, he's nearly doubled it to 19.9 -- the 26th-highest mark in the Majors.
Alonso's slugging percentage is up 189 points over last season, and his wRC+ is 158, good for fourth among first basemen with at least 70 plate appearances. He's already hit five home runs. It took him until Aug. 2 to do that in 2016.
It's not the only difference in Alonso's swing. Here's his approach last season, when he used a small toe tap before lacing a line-drive single:

Compare that to now, as Alonso attacks pitches with a large leg kick and an uppercut swing. You can't golf many pitches out of the park like this without getting a good amount of air under it:

Though Alonso lacks Zimmerman's elite exit velocity, the change has already made a drastic difference in the quality of contact off the bat. His 2016 zone chart looks like something out of Picasso's blue period, while '17 is a cubist masterpiece loaded with blazing red:

Of course, there is still reason for concern. Alonso's strikeout rate is a career-high 23.8 percent -- though in today's strikeout-friendly game, that's a welcome tradeoff if the newfound power is for real. The A's are also protecting the hitter against left-handers, limiting him to just 13 plate appearances -- even if he has already matched last year's dinger total off southpaws with one.
After signing a one-year deal for $4 million in the offseason, Alonso is not only one of the best value signings of the offseason, but he could be a valuable trade chip as teams hunt for left-handed power at the non-waiver Trade Deadline. And that's not something you could have said coming into the season.