Borkowski and Castillo have the right to refuse the assignment and become free agents. Cassel and Houston become free agents because of their minor league service time.
Borkowski was 0-2 with a 7.50 ERA in 26 games for the Astros in 2008. Cassel was 1-1 with a 5.64 ERA in nine games, including three starts, for the team last season.
Houston was 1-3 with a 7.54 ERA and one save in 39 games for Round Rock in 2008. He was claimed off waivers from Toronto on Dec. 12.
Castillo hit .246 with six home runs and 37 RBI in 127 combined games for San Francisco and Houston last season. He came to the Astros from the Giants on Aug. 20.