In Zach Levine's blog post last night we read that the Astros will try to exhaust their options for SP5 before looking elsewhere. Maybe.
Wade, speaking on behalf of Brad Arnsberg, said that Arnsberg can get a pitcher ready to start over the course of one Spring Training.
Within that link, Levine breaks down the chances of Figueroa, Lyles, Abad, and Villar of becoming that SP5.