With Clay Buchholz pitching a strong eight innings in the Red Sox's 2-1 victory over the Blue Jays, Castro was not needed and found out after the game that he was on his way back to Pawtucket.
"[Embree will] be here [Wednesday]," manager Terry Francona said after Tuesday's game. "We needed Castro here in case something happened. Now we can get a reliever. We have a day off the next day [Thursday]. I think we'll have some semblance of order after tonight and we should be OK."
Embree, who was a member of Boston's bullpen from 2002-04, is 0-0 with a 3.68 ERA in eight games at Triple-A.
Boston was forced to make a roster prior to Tuesday's game because its bullpen had logged 9 1/3 innings over the previous two games -- a 10-inning 7-6 loss to the Orioles on Sunday and a 13-12 win over the Blue Jays on Monday.
In order to make room on their roster for Castro, who was slated to start Tuesday for Pawtucket, the Red Sox optioned right-hander Scott Atchison to Triple-A.
Atchison (0-1, 6.10 ERA) was the loser in Sunday's game and also pitched 1 2/3 innings on Monday.
Tim Wakefield, who went 6 2/3 innings in a start on Sunday, will be moved to the bullpen to make way for Daisuke Matsuzaka in the rotation, but the knuckleballer will not be available until Friday, when the Red Sox open a three-game series in Baltimore.
Castro, 25, has made three starts for the PawSox this year, going 0-1 with an 8.10 ERA.
Castro is in his first season in the Red Sox's organization after signing as a free agent in December.
He split the 2009 campaign between Toronto's Double-A New Hampshire and Triple-A Las Vegas affiliates, combining to go 9-6 with a 4.01 ERA. The left-hander last appeared in the Majors with Philadelphia in 2007. He is 0-1 with a 3.30 ERA in 30 career games, including one start, for the Rangers in 2006 and Phillies in 2006-07.
Larry Millson is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.