It's taken a while for 2008's 7th-overall pick to figure things out at baseball's highest level, but Yonder Alonso is in the midst of a breakout campaign in Oakland. This year, the 30 year-old first baseman has a solid .264/.362/.531 triple-slash and 21 home runs (139 wRC+). Alonso is eligible for free agency this offseason, so the timing of his best season to date couldn't have been better for himself. Though Oakland and Alonso are mutually interested in an extension, the first baseman has been subject to plenty of trade rumors given Oakland's usual reluctance to pay up in free agency. The Yankees have been connected to Alonso for much of the season, and now with the trade deadline less than a week away, it appears that Alonso could be included in a trade to the Bronx that Sonny Gray would headline. The Yankees could still use some help at first base, but what is Alonso actually worth?
Plenty has been written about the legitimacy of Alonso's breakout season. In the past, Alonso has struggled to lift the ball, but this season, he's selling out for fly balls and it's certainly paid off. At first glance, it doesn't sound like this season has been a fluke, as his improvement can be pointed at an actual adjustment he made at the plate. Further, as a former high draft pick and top prospect, it's not like Alonso wasn't inherently talented. Sure, it took him a while to figure things out, but perhaps it's not a total surprise given his early career expectations. All that being said, there are some aspects of Alonso's season that are troubling and cast some doubt on his viability going forward.
Any hitter that improves his fly ball rate from 33.3% a year ago to 46.4% this season is going to hit more home runs. That's precisely how Alonso's rates have changed. However, his home run to fly ball rate is quite high, standing at 21.4%. League average stands at 13.7%, and Alonso has never posted a full year rate higher than 7.8%. So while I can buy that Alonso has turned himself into a fly ball hitter due to a mechanical change, I'm not sure I buy into the frequency his fly balls are going over the fence. According to Stacast, one piece of data on Alonso's side is that his average exit velocity on fly balls is up to 93.7 MPH this season, much higher than 90.3 MPH a year ago. Even so, it's such a significant jump from his career norms that I'm still skeptical.
May was by far Alonso's best month, but the rest of his season hasn't been overwhelming. During May, Alonso clubbed 10 home runs in 80 plate appearances and slashed a ridiculous .303/.425/.803 (217 wRC+). In the months surrounding, Alonso still has been pretty good but not superb. April was his second best month, when he hit .279/.355/.515 (133 wRC+) with 4 home runs in 76 opportunities. After a strong start, Alonso's not done much since the calendar turned to June. He's been slightly above average at the plate, hitting .241/.337/.424 (107 wRC+) while swatting 7 long balls in 181 plate appearances. Furthermore, his HR/FB rate has come back to earth (15.6%). The June and onward version of Alonso is still a pretty good hitter, but not much different than the guy he was entering 2017. His career wRC+ entering this season was 102, based on a .269/.334/.387 line. The version we've seen over approximately the last two months is essentially that with some additional power.
Moving forward, projection systems believe that Alonso is much more like his June through July self. ZiPS and Steamer forecast 109 and 110 wRC+, respectively. Both expect decent power, with ZiPS the high system, expecting a .172 ISO vs. Steamer's .166 estimate. That's a good projection, but it definitely pumps the breaks on Alonso's full season line.
Despite his former prospect status and mechanical adjustments, I'm having a hard time buying into his full season stat line being representative of Alonso. Based on what he's done since June and what the projections indicate, it appears that Alonso is a slightly above average hitter. Given that he'll be a rental, I don't think that makes him worth more than one or two low-level prospects. I hate making hypothetical trade proposals because they often sound stupid, but let's put myself at risk anyway: for Alonso only, could Hoy Jun Park and/or Dermis Garcia get it done? Feel free to ridicule that for being too much or too little. One problem is that if Alonso is lumped into a trade with Gray, it'll be difficult to bifurcate the additional prospect(s) that would net Alonso into the deal.
If the Yankees are dead set on acquiring another first baseman, Alonso is a fine option. He would be a boost over Chase Headley and Garrett Cooper at first base, and in combination with the recent acquisition of Todd Frazier, Alonso would make the Yankees' corner infield better than it was just a couple weeks ago. I'm still wary of Alonso's supposed breakout, but if the prospect cost is reasonable, he makes sense (obvious statement is obvious).
Statistics via Fangraphs. Statcast data via Baseball Savant.