It might be a little early to do a retrospective on a trade that only just happened last offseason, but I think enough has happened to at least judge the deal on the first year. Anyone who has paid attention to the Yankees, or even much of baseball, should be aware about how much of a victory the trade for Didi Gregorius turned out to be for Brian Cashman and company. Maybe things change over the next few years, but right now things are looking good for the Yankees.
After Derek Jeter retired, the Yankees needed a new shortstop, someone who was young, athletic, and good defensively. In a surprise three-team deal, New York sent right-hander Shane Greene to the Detroit Tigers, the Tigers sent Domingo Leyba and Robbie Ray to the Arizona Diamondbacks, and the D-Backs sent Gregorius to the Bronx.
At the time a lot of Yankees fans were unhappy with the deal. Greene had just come off a very promising rookie season in 2014 where he pitched to a 3.78 ERA and 3.73 FIP in 78.2 innings. He managed to be worth 1.1 WAR and, most impressively, maintained a 50% ground ball percentage. After several years as a fringe prospect, it looked like Greene finally flourished after being given a chance at the major league level–something that didn't seem all that possible when the season began.
Gregorius, on the other hand, had been around for a few years and, despite his youth, was already losing some of his sheen. He had already been traded from the Reds to the Diamondbacks in a different three-way deal before the 2013 season and had disappointed in Arizona. At the time of his trade to the Yankees, he was coming off a season where he had hit .226/.290/.363 and very much seemed like the type of guy you were always waiting for to take off. You can see why at the time of the trade it looked like the Yankees had given up on Greene way too soon for a player who would never amount to much, but we were all wrong.
In 2015, after a disappointing April, Didi took off and did things he had never managed to do before. He finished the season with career highs in every category, batting .265/.318/.370 with nine home runs. He also exceeded in the field and advanced metrics finally graded him as an above-average fielder, all amounting to a 3.1-WAR season that most of us never saw coming. He was the most valuable position player and ranked third behind Michael Pineda and Nathan Eovaldi in terms of value on the entire roster. He will never be anything more than a league-average hitter at best, but shortstops like Jeter are mostly a thing of the past, so this is the best we can hope for. He'll only be 26 in 2016 and, since he's a Super Two, won't be a free agent for another four years.
Shane Greene, on the other hand, had a disastrous debut for his new team. In his first three starts with Detroit he had a combined 11 strikeouts and five walks while allowing only one earned run on 12 hits in 23 innings. Suddenly, the promising young pitcher that we gave away for the future at shortstop looked like a future ace. But that all ended quickly. He struggled mightily from there, often getting blown out by the fourth inning. By the beginning of June, the Tiger had seen enough and they optioned him to the minors to get things right. He seemed to be fine at Triple-A, so he was eventually recalled a month later, but everything was the same and he was even briefly moved to the bullpen.
Unfortunately, Greene's season turned from bad to worse when he was diagnosed with an aneurism in his throwing shoulder. Whether that had to do with his struggles is unknown, but he underwent surgery in August. He finished the season with a 6.88 ERA, but if you remove those three good starts at the beginning of the year, the number jumps to 9.35 on the year. He is currently on the road to recovery over the winter, but following the additions of Daniel Norris and Mike Pelfrey, it's likely Greene will start the season off in Triple-A.
As for Arizona's return for Didi, Robbie Ray–from the infamous Doug Fister trade–pitched well for his new team with a 3.52 ERA and 3.53 FIP in 127.2 innings, despite spending the first two months of the season in the minors. He was worth 2.1 WAR, which is pretty solid, and probably makes him pretty comparable to what Shane Greene did for the Yankees in 2014, only over a larger sample. Prospect Domingo Leyba stuck it out at shortstop, despite 20 errors on the season, but hit an ugly .237/.277/.309 over 562 plate appearances at High-A. He currently ranks among their top 10 prospects, but I'm not sure if that's good for the trade or bad for their system.
If I were to rank each team by how well off they are by this trade, I would say Yankees > Diamondbacks > Tigers, but the addition of Ray for the Diamondbacks makes this pretty close. Luckily, the Yankees gave up Greene and not Ray, so this trade, if we take it as Greene for Didi, is a complete steal. Of course, this is only year one of the deal, both players will be with their new teams for a long time, and plenty can change. The Yankees are just in the lead right now.