Castro was called up earlier in the day to provide extra depth for Boston's bullpen after six Red Sox relievers combined to pitch six innings in Monday's 13-12 victory. But he wasn't needed in Tuesday night's 2-1 win over the Blue Jays.
Right-hander Scott Atchison was demoted to make room for Castro, 0-1 with an 8.10 ERA in three starts at Pawtucket.
Atchison was 0-1 with a 6.10 ERA in seven relief appearances this season, his first with Boston.
Embree was 0-0 with a 3.68 ERA in eight games at Triple-A. Due to a clause in Embree's contract, the Red Sox faced a Friday deadline to recall Embree or the reliever could have walked away from the team.
Entering play Tuesday, Boston relievers had thrown 70 innings this season, the most in the American League.
Wakefield would be ready to pitch in relief Friday, manager Terry Francona said. The Red Sox are off Thursday before opening a three-game series at Baltimore.
"That way it's almost like a normal week for him with his off day on Thursday," Francona said. "That kind of slots him in where it works."
Wakefield, 0-1 with a 5.40 ERA in four starts, last pitched Sunday at home against the Orioles, allowing two runs and seven hits in 6 2/3 innings.