The St. Louis Cardinals wanted outfield help and they got it, trading from their two organizational strengths of outfield and starting pitching prospects, while adding a somewhat high-beta outfielder in Marcell Ozuna who could make them several wins better by himself. For the Miami Marlins, it's the best baseball move they've made this winter, but that's not saying a whole lot.
Ozuna is coming off by far his best year as a big leaguer, hitting .312/.376/.548 with 37 homers and 60 unintentional walks -- all career highs -- while playing above-average defense in left field. The Cardinals saw a huge breakout from a finally healthy Tommy Pham in 2017, but got very little from their other corner outfielders, with Randal Grichuk continuing to struggle to reach base and Stephen Piscotty losing all of his power. If Ozuna repeats his 2017 line or comes fairly close, he's an easy two-win upgrade over whoever he's replacing, with a chance for more.
It's worth remembering that Ozuna had a .321 OBP in 2016 and .308 OBP in 2015, and that his walk rate in 2017 was well above anything he'd posted in the majors before (and even higher than his best walk rate in the minors), so it is far from certain that the level of production he showed in 2017 is his new baseline rather than a single-year fluke. He's a free agent after 2019, probably headed for a large raise this winter in arbitration.
In exchange, the Marlins didn't do well, but they didn't do as poorly from a baseball perspective as they did in the salary dumps of Giancarlo Stanton and Dee Gordon. Right-hander Sandy Alcantara was among the Cardinals' best pitching prospects, but isn't yet ready for a major league rotation spot, promising huge upside but with some bullpen risk. He has been clocked throwing 95-100 mph as a starter and shows four pitches, barely using a changeup but flashing both a hard upper-80s slider that touches 90 and a sharp but inconsistent curveball at 79-84 mph. He's listed at 6-foot-5 and probably still under 200 pounds, although he still has some room to fill out.
For as hard as Alcantara throws, he gives up a lot of hard contact, and he's going to have to develop his changeup more and also get at least one breaking ball to above-average to be able to start. But he can do all of that, showing enough already to put that within the realm of possibility. Alcantara could end up a No. 2 starter, but there's legitimate risk he never gets close to that, ending up as an Edwin Jackson sort of back-end starter, or perhaps going to the bullpen if he can't miss enough bats as a starter.
Magneuris Sierra played briefly in the majors in 2017. He’s a potentially game-changing defender in center with plus speed and good contact rates, unlikely to ever hit for power but otherwise a potential everyday player. His chance to become more than that depends more on his willingness to work the count; he puts the ball in play often, but rarely runs deep counts and thus never walks, which severely limits his OBP and thus his leadoff potential.
Right-hander Zac Gallen, the Cardinals' third-round pick in 2016, has started in the minors but lacks an out pitch and is likely to end up in the pen or as a fifth or sixth starter. Daniel Castano, an organizational starter, was a senior sign out of Baylor in 2016 and has yet to reach full-season ball at age 23.
That amounts to a likely rotation member with a very wide range of outcomes, a future everyday center fielder, and maybe two relievers if they're fortunate. For two years of Ozuna, it seems a little light, given what Ozuna was able to produce last season; the Cardinals may have just acquired about eight wins above replacement over two seasons without dealing their top pitching prospect or top outfield prospect, and the lesser players in the trade aren't guys with upside who are just really far away.