Rosenbaum, 27, began last season with the Nationals' Triple-A Syracuse affiliate and made four starts, going 1-1 with a 4.50 ERA (10 ER/20.0 IP), but missed the remainder of the season after undergoing Tommy John surgery on May 8. In 2013, his last full season and his first year at Triple-A, Rosenbaum tied for the International League lead with a career-high 28 starts and also ranked fifth in the IL with 158.1 innings. He posted a 3.87 ERA (68 ER), including a 2.95 mark (13 ER/39.2 IP) in his last seven starts over August and September.
The left-hander was a Rule 5 Draft selection by the Colorado Rockies in December 2012 but was returned to Washington during the following spring training. Born in Loveland, OH, Rosenbaum was named to mid-season All-Star teams in the Single-A South Atlantic League in 2010 and the Double-A Eastern League in 2012.
Signed by Washington as a 22nd-round selection in the 2009 June Draft, he is 34-36 with a 3.12 ERA (238 ER), 498 strikeouts, and only 213 walks allowed in 686.0 innings over 121 career minor league games, including 116 starts. He has induced nearly twice as many ground outs (956) as air outs (508) in his pro career, allowing only 31 career home runs.
Butler, 28, had been designated for assignment last week. He went 4-for-19 (.211) with three doubles, two RBI, and a walk in seven games for the Red Sox over two stints with the club in his major league debut. The right-handed batter spent the majority of 2014 with Triple-A Pawtucket and appeared in 83 games. He led the PawSox with 54 games at catcher and threw out 16 of 60 attempted base stealers (27%). Signed as an undrafted free agent in 2009, Butler has hit .256 (431-for-1,682) with 115 doubles, six triples, 47 home runs, and 241 RBI in 497 career games over six minor league seasons.