Butler made his Major League debut with Boston last year, and he went 4-for-19 (.211) with two RBIs. Before reaching the Major Leagues, he spent six seasons in the Red Sox's farm system, hitting .256 with 47 home runs and 241 RBIs. Butler is expected to be with the Nationals during Spring Training.
Rosenbaum spent six years in the Nats' organization, and he missed most of the 2014 season because of Tommy John surgery. His best season was in 2011, when he went a combined 9-6 with a 2.52 ERA for Class A Advanced Potomac and Double-A Harrisburg.
Rosenbaum left a message on Twitter (@drosenbaum10), thanking the Nationals for the six years.
"Can't thank the @Nationals enough for everything! Now starts a new journey and opportunity with the @RedSox."
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.