He finished the 2006 season with a 1.54 ERA. Castro started the 2007 season with the Ottawa Lynx, the Triple-A affiliate for the Phillies. On April 26, 2007, Castro was called back up to the Phillies when the team optioned pitcher Matt Smith for assignment. On August 28, 2008, Castro was called up to the Phillies, replacing Andrew Carpenter. He was optioned the following day, without making an appearance. Castro was traded to the Toronto Blue Jays at the end of the 2008 season to complete a trade for Matt Stairs. Castro signed a minor league contract with the Boston Red Sox on December 7, 2009.