Catcher Sandy Leon was also selected from Pawtucket, while right-hander Noe Ramirez was optioned to Triple-A.
For Castillo, 28, this will be his third Major League stint in 2016. He is 2-for-4 with a double in two games for Boston this season. Hembree, 27, returns to the Red Sox for his second stint in the Majors this season. He is 2-0 with a 2.14 ERA and 18 strikeouts in 12 Major League appearances.
This will mark the first Major League time the 27-year-old Leon will see this season. He was hitting .243 with three doubles and two home runs in 29 games for Pawtucket.