Bostick was acquired by the Nationals from the Texas Rangers, along with right-handed pitcher Abel de Los Santos, in exchange for left-handed pitcher Ross Detwiler on December 11, 2014. He began the 2015 season with the Potomac Nationals before being promoted to Harrisburg on June 14. Bostick enters the 2016 season rated as the No. 23 prospect in Washington's system, according to MLB.com.
Kieboom, 24, was rated as the top defensive catcher in the Carolina League, according to Baseball America. He has thrown out 41 percent (77-for-189) of would-be base stealers over his three-year Minor League career. Throughout his professional career, Kieboom has been tasked with handling many of the top pitching prospects in all of Minor League baseball, most notably Lucas Giolito, Reynaldo Lopez, Erick Fedde, and Austin Voth, among others. He was originally selected in the fifth round of the 2012 First-Year Player Draft out of Clemson (SC) University.
In 71 games for Single-A Potomac in 2015, Kieboom hit .248 with 16 doubles, one triple, two home runs, 26 RBI, 36 walks and 30 runs scored. He was named a Carolina League mid-season All-Star after earning South Atlantic League All-Star honors in 2014. Kieboom hit .238 (10-for-42) with two homers, seven RBI, eight walks and three runs scored in 13 games for the Salt River Rafters of the Arizona Fall League.
Lee, 24, made the transition from starting to relieving in 2014 and has continued to thrive in that role, striking out over one batter per inning (271 strikeouts in 249.0 IP) while maintaining a career 9.8 strikeouts per 9.0 innings ratio. Since the start of 2014, 42 of Lee's 51 appearances as a reliever have been at least one inning in length. In 2015, he ranked tied for second in Washington's chain with 10 saves while going 3-1 with a 3.12 ERA in 40 games between Single-A Potomac and Double-A Harrisburg. He surrendered just one home run while holding opposing batters to a .216 average against him.
A former 18th-round pick out of Weatherford (TX) Community College in the 2011 First-Year Player Draft, Lee began the 2015 season with Potomac before earning a promotion to Harrisburg on June 30th. From August 11 through the end of the season, Lee posted a 0.73 ERA (1 ER/12.1 IP) while striking out 14 batters in nine appearances for the Senators. At the conclusion of the regular season, Lee played for the Salt River Rafters of the Arizona Fall League, appearing in six games (6.2 IP), going 0-1 with a 4.05 ERA.
With these moves, Washington has 38 players on its 40-man roster.