Herrera, 28, has pitched to a 1.05 ERA and 11.00 K/BB ratio in 25 2/3 dominant innings for the Royals this season. The two-time All-Star for his career owns an ERA+ of 153 across parts of eight big-league seasons, all with Kansas City. Herrera is owed the balance of a $7.94 million salary for remainder of 2018 and is eligible for free agency this coming winter.
In D.C., he'll join a Nationals bullpen that ranks sixth in the NL in relief ERA and seventh in relief FIP. Herrera figures to form a formidable lefty-righty combo with current Nats closer Sean Doolittle. More generally, the Nationals are all-in on trying to make it deep into the postseason in Bryce Harper's walk year, so don't be surprised if GM Mike Rizzo makes additional moves before the July 31 non-waiver trade deadline. By striking early in this instance, however, Rizzo gets Herrera for an extra six weeks or so.
On the other side, the Royals receive minor-league infielder Kelvin Gutierrez, minor-league outfielder Blake Perkins, and minor-league right-hander Yohanse Morel. Gutierrez, 23, has batted .285/.344/.388 across parts of six minor-league seasons with 22 triples and 55 stolen bases in 416 games. He's a primary third baseman who's also seen time at short and first. This season, he's been at Double-A Harrisburg. Perkins, 21 and a former second-rounder in 2015, has a .655 OPS in four partial minor-league seasons. He's a center fielder who's been at the High-A level this season. Finally, Morel is just 17 years of age, and he's made only one start in the Dominican Summer League in this, his first pro season. According to MLB Pipeline, Gutierrez and Perkins were the Nationals No. 10 and No. 11 prospects, respectively.
The Royals of course just recently traded away outfielder Jon Jay, and as such they could be entering full-on sell mode.