Castro began this past season at Nashville, posting a 3.32 ERA in 33 appearances (38 innings). After being promoted to the A's midseason, he posted a 4.38 ERA in 26 appearances (37 innings).
Smith also made 26 appearances for Oakland in '17, posting a 4.89 ERA (35 innings). At Nashville for part of the season, he posted a 3.70 ERA in 19 appearances (two starts).
Manny Randhawa is a reporter for MLB.com based in Denver. Follow him on Twitter at @MannyOnMLB. This story was not subject to the approval of Major League Baseball or its clubs.