It only made sense for the Arizona Diamondbacks to trade Paul Goldschmidt, who is one year from free agency and whose contract pays him just $14 million this year, making that year quite valuable in the trade market to any team looking to contend in 2019 and with an open spot at first base. The St. Louis Cardinals were a perfect trade partner, with surplus talent behind the plate and on the mound -- and in the outfield, where they still have a few more players than they can use -- although adding Goldschmidt creates a defensive liability across the diamond.
Goldschmidt had another Paul Goldschmidt year in 2018, despite an extended cold stretch that had him hitting .210/.325/.390 on June 1. He hit .330/.420/.602 the rest of the season and ended up over 5 WAR for the fourth year in a row and fifth time in the past six years. How much this benefits the Cardinals depends on who loses at-bats in the exchange -- Jedd Gyorko seems the most likely odd man out, entering the last year of his deal. With $8 million due to him from the Cardinals (the rest paid by San Diego), Gyorko, who was worth about 2 WAR in a utility role last year, should have some value in trade as a lefty masher who can play second base or third base comfortably.
Matt Carpenter thus returns to third base, where he hasn't played full time since 2015 and was never even an average defender, so there's some risk here if he goes to the hot corner and it turns out age has made him a major liability. Goldschmidt's offense, defense, baserunning and salary make the trade worth that risk, though; the Cards are likely three wins better right off the bat, perhaps more if Goldschmidt hits like himself for the entire season. There is the possibility that Carpenter, now 33 and never a good defender, won't move well enough to handle third on a daily basis, and in that case the Cardinals will have a problem they can't easily solve.
Arizona gets one top prospect in return, two other players and a valuable draft pick. Catcher Carson Kelly was blocked in St. Louis by Yadier Molina and the Cardinals' apparent unwillingness to transition Molina out of a full-time role. They were able to deal Kelly because Andrew Knizner has emerged as a legitimate catching prospect in his own right and is close to major-league ready. Kelly is definitely major-league ready and should step in as the primary catcher for the Diamondbacks in 2019.
Converted from third base about a year after the Cardinals drafted him, Kelly developed into an above-average defensive catcher with a plus arm and above-average framing and receiving skills, as well as a reputation for a great work ethic and talent working with pitchers. His bat has lagged, in part because his swing has become so flat that the natural power he has is wasted. In his 131 major-league plate appearances to date, he has posted a 56 percent ground ball rate, which would be a fatal flaw, except that it's way above what he did in the minors. He could be a soft regular with just a little more contact in the air, but there's above-average or better upside if he can improve that launch angle and get to the average or slightly better raw power that he has.
Luke Weaver is a back-end starter who might be better used as a multi-inning reliever now that that role is back in vogue. He'll sit 92-95 as a starter with an above-average to plus changeup, but has never had anything like an average breaking ball, so getting through a lineup three times is a problem, even if it's heavily right-handed. His arm is very quick, but the Cards never found a slider that worked for him.
Andy Young was a 34th-round pick in 2016 who has had great contact rates since signing, posting a .379 OBP between High-A and Double-A this year as a 24-year-old and making consistent hard contact through a stint in the Arizona Fall League. The Indiana State product played mostly second base this year but has had reps at third and short in the past two years. He looks like at least a utility infielder, but the frequency and quality of contact seem to point to him as a player with upside even at his age.
The Diamondbacks already had four of the first 75-80 picks in next June's draft, then added another pick when Patrick Corbin signed with Washington, and they may get one more depending on where and for how much A.J. Pollock signs. The fourth part of this trade nets them yet another pick, in the Competitive Balance B round (between Rounds 2 and 3), where picks last year carried slot values of $812,000 to $894,600. This is a long way of saying that Arizona is going to have a ton of money in their bonus pool in June, with two picks in the first round at 16 and 26 (as of now), and thus the flexibility to potentially move money around and sign some first-round talents with picks after the first round because they'll have the money to go over slot. The Diamondbacks are trying to rebuild quickly, given the talent already in the system from the past few years; their 2019 draft class should infuse a significant number of high-ceiling players into the organization to further speed that process along.