The Marlins did a good trade! They got three legitimate prospects back for Christian Yelich, and the fourth isn't nothing, which makes this the best trade they've made in this cash-grab offseason.
Lewis Brinson, the No. 32 prospect in baseball for 2018, is a plus-plus defender in center with great bat speed and big raw power, with staying healthy his main problem so far in pro ball. He should be able to step in immediately to play center in Miami, which would also allow the Marlins to send Magneuris Sierra to the minors to continue developing as a hitter. Brinson's swing can get long, and he's going to strike out at a fairly high rate, but the ball comes off his bat extremely well and I think he'll more than make up for the whiffs with extra-base power.
Monte Harrison, the No. 85 prospect, had a breakout season as a prospect in low-A and high-A last year, shortening his swing to make better quality contact and more of it. He's a plus runner who can play all three outfield spots, and has the power for a corner, with 21 homers last year, setting a new career high. His baseball instincts still aren't great, as he was a football recruit to the University of Nebraska after starring in multiple sports in high school, so there may be further growth to come and there's still some risk that he doesn't get to enough of his power because of the strikeouts.
Isan Diaz's year was disappointing after he slugged .469 and hit 20 homers in low-A in 2016, although he played with a hamate injury for part of 2017, eventually having surgery to repair a fracture at the end of August. He has played shortstop in pro ball but he's not a regular there, with much better defense at second base and the power to profile there. He did play much better in the Pan Am 23U qualifiers in November, showing glimpses of power again, and the Marlins may have done well to acquire him now while his value was temporarily low.
Jordan Yamamoto is a command and control right-hander who pitched very well in high-A last year at 21, but at just 6 feet tall he doesn't get any plane on his fastball, making him fly ball and potentially homer-prone. He comes from a lower slot that might pose platoon issues at higher levels, although that was not an issue in 2017.
In total, that's two top-100 prospects plus a former top-100 prospect, or a potential above-average everyday center fielder plus an above-average outfield prospect plus a solid-average or better second-base prospect plus a potential middle reliever in exchange for Christian Yelich and his very team-friendly contract. That seems like a good haul for the Fish, and very much what they needed after several deals that didn't bring back enough baseball talent.
The Brewers got a great player on a great contract, though, so even though they gave up quite a bit of prospect value, I still think they did well in the deal. Yelich has averaged 4 WAR per year since his first season as a major league regular, and would be an instant three-win upgrade over Keon Broxton in center. Milwaukee wasn't that far out of contention last year, and while the Brewers still have a few holes to fill -- first base was a weak spot after April, they could definitely use another starter -- this was a sensible deal that helps them compete again in 2018 but has a lasting benefit well beyond this season, extending into the period where the Brewers' top remaining prospects like Corbin Burnes, Keston Hiura and Lucas Erceg will all be on the major league roster as well.