Hairston, 33, received limited playing time in right field after inking a two-year, $5 million deal during the offseason. Cubs manager Dale Sveum used Hairston to spark the Cubs offense against left-handers this season and the Nationals look to do the same. He batted .179 against lefties and .172 overall with eight home runs and 18 RBIs this season.
Pineyro, 21, is in his third Minor League season and started this year with Class A Hagerstown. He recorded a 5-3 record in 13 starts with a 3.14 ERA before joining Potomac where he was 1-0 in two starts.
Hairston joins Scott Feldman, Carlos Marmol, Steve Clevenger as Cubs dealt to gain minor league prospects before the July 31 Trade Deadline.