Ozuna, 27, is coming off an excellent 2017 in which he slashed .312/.376/.548 (145 OPS+) with 37 home runs in 159 games. In trading for Ozuna, the Cardinals are betting that he's capable of repeating such a breakout performance, as he had a career OPS+ of 103 coming into last season.
When it comes to sustaining his 2017 levels, things might not be promising on that front. To get an idea of this, we'll turn to an advanced metric called expected weighted on-base average (xwOBA). xwOBA grows out of wOBA, which assigns proper value to every possible offensive event that happens while a batter is at the plate. Those proper valuations of singles, doubles, homers, walks, etc., distinguish wOBA from more traditional measures like AVG, OBP, and SLG. Also, for simplicity wOBA is scaled to look like OBP, which means that, say, .400 is elite and .290 is pretty poor. For instance, Babe Ruth is the all-time leader with a patently absurd wOBA of .513.
All of that brings us back to xwOBA, which is an estimation of what a hitter's wOBA should be based on things like exit velocity off the bat and launch angle. xwOBA attempts to strip away luck -- bad or good -- and defensive play from wOBA and identify a hitter's baseline skill. It's useful for getting an idea of how a hitter figures to perform in the near-term future. Basically, if a hitter's xwOBA is significantly lower than his wOBA, he's probably going to come back to earth at some point. There's some evidence that slower players tend to underperform relative to their xwOBA and faster players tend to overperform, but even so xwOBA has utility. On the other side of things, if a hitter's xwOBA is quite a bit higher than his wOBA, then better days are likely ahead. Here's more on wOBA, and here's more on xwOBA.
In Ozuna's case, he put up a robust wOBA of .397 last season, which is an excellent figure. His xwOBA, however, checks in at .359, which is good but not great. That raises some concerns about regression moving forward. It's of course possible that the Cardinals -- being, you know, a major-league organization -- have better ways to measure such things and are confident in what their internal measures tell them. All that said, there's some risk with Ozuna based on his relatively brief time as a high-level producer.
On the contract front, Ozuna is second-year arbitration-eligible this offseason, and he's not eligible for free agency until after the 2019 season. MLB Trade Rumors projects him to earn $10.9 million in 2018.
As for the Marlins' side of things, it's expected they'll be receiving a four-prospect package led by right-hander Sandy Alcantara.
Source: In addition to Alcantara and Sierra, other two prospects Marlins receiving in Ozuna deal are rhp Zac Gallen and lhp Daniel Castano.— clarkspencer (@clarkspencer) December 13, 2017
The 22-year-old Alcantara made eight relief appearances for the Cardinals last season. He has elite fastball and sinker velocity and also mixes in a hard slider. If Alcantara continues making progress with his changeup, then he may have a future as a rotation cornerstone (of his 72 minor-league appearances, 68 were starts).
The Marlins will also reportedly acquire outfield prospect Magneuris Sierra, a slick defender with some offensive upside as a slash-and-dash type. He appeared in 22 games for St. Louis in 2017. Gallen had a 2.93 ERA in 147 2/3 innings this past season and reached Triple-A. He is a scouts' favorite thanks to his deep arsenal, his pitchability, and his competitiveness. Castano is a depth arm who threw 91 innings with a 2.47 ERA in the low minors this summer.
On another level, the continuing sell-off means the Marlins must decide whether outfielder Christian Yelich, who's signed long-term to a bargain contract, is worth building around or should be flipped for more prospects. Given his upside and years of control, Yelich would likely fetch more in return than Ozuna or Stanton.