Roark went 9-15 with a 4.34 ERA and 146 strikeouts in 180.1 innings last season.
Mark Zuckerman of MASN Sports gave some background on Rainey:
Roark has experienced a bit of a roller-coaster career, with a sub-3.0 ERA in 2014 and 2016 and marks of 4.34 or higher in 2015, 2017 and 2018. Notably, his 2014 and 2016 seasons played a significant part in the Nats winning the NL East with 96- and 95-win campaigns, respectively.
Per FanGraphs, Roark's fastball velocity has noticeably trended in the wrong direction in recent years, going from 93.7 mph in 2015 and down to a career-low of 91.9 mph in 2018. A four-pitch pitcher, the 32-year-old throws a slider, curveball and change-up as well.
Roark joins a Reds starting rotation that struggled in 2018. Six pitchers made 20 or more starts, and only two (Matt Harvey and Luis Castillo) had ERA lower than 4.93. As a team, the Reds were second-last in the National League in ERA at 4.63.
While Roark's recent numbers aren't spectacular, he should at least provide stability to a Reds team in significant need of starting pitching help.
As for the Washington Nationals, their significant franchise overhaul continues. The Nats are building around their starting rotation, which now features three at-times dominant pitchers in Max Scherzer, Stephen Strasburg and newly acquired Patrick Corbin.
Roark could have started on the back end of the rotation alongside Joe Ross, but that won't be the case anymore. Chelsea Janes of the Washington Post explained what this means for the Nats moving forward:
Entering his age-32 season, Roark is a free agent in 2020. Rainey isn't his likely immediate replacement given that he's never made an MLB start and had a 24.43 ERA in eight relief appearances for the Reds last year, but he's entering his age-26 season and could certainly provide some help for the Washington bullpen with further development.