The move is surprising because Chapman is under investigation by Major League Baseball for an alleged domestic violence incident that involved gunshots and the choking of his girlfriend. No criminal charges were filed, but that does not mean Chapman will not be suspended under MLB's domestic violence policy. One might argue a suspension is likely, in fact, and that cloud of suspicion nixed a trade of Chapman to the Los Angeles Dodgers in early December.
Given the circumstances, it was assumed that Chapman would be impossible for the Reds to trade, but the Yankees are willing to take the risk. There are three reasons why New York would make this deal:
1. The price in prospects was not high. The Reds acquired right-handed pitchers Caleb Cotham and Rookie Davis, plus infielders Eric Jagielo and Tony Renda. Raise your hand if you've heard of any of them. I have not. These are not blue-chippers. The Yankees did not have to give up any of the crown jewels of their farm system to acquire a 27-year-old closer who has accumulated 145 saves over the past four seasons.
2. The Yankees now have a devastating back end of the bullpen that rivals, and perhaps betters, that of the world champion Kansas City Royals. Baseball's top three relievers in strikeouts per nine innings during the 2015 campaign were Chapman (15.74 Ks. per 9 IP), Andrew Miller (14.59) and Dellin Betances (14.04). All of them now play for New York. If all three of those guys are healthy and effective, how many games do you suppose the Yankees are going to blow in the seventh, eighth or ninth inning? Probably not too many.
3. New York also might luck into an extra year of team control with Chapman. Right now, the left-hander is scheduled to become a free agent after the 2016 season. But, let's say he is suspended for 40 games for the domestic violence incident. He would lose that service time, and accordingly, his free agency would be delayed until the conclusion of the 2017 season. Sure, the Yankees would lose him during the suspension, but they'd still have him closing games for the next season and a half once the suspension is over.
This trade isn't the most PR-savvy thing for the Yankees to do. They will be rightfully criticized for acquiring a player with as much off-the-field baggage as Chapman, but you can see why this move could be a steal from a purely baseball perspective.