Borkowski, 31, has posted a 1-1 record and a 2.29 ERA in 12 appearances with the Express. Borkowski began the season in the Astros' bullpen and was 0-2 with a 6.86 ERA in 18 appearances. He is 8-7 with one save and a 5.17 ERA in 122 appearances for Houston over three seasons from 2006-08.
The 26-year-old Villarreal went 1-3 with a 5.02 ERA in 35 appearances for the Astros this season. The right-hander owns a career 24-15 record and a 3.86 ERA in six Major League seasons. Villarreal was acquired prior to the 2008 season from Atlanta in exchange for outfielder Josh Anderson.
Borkowski has been added to the club's 40-man roster, which stands at 36 players, including 20 pitchers and 16 position players. The club continues a 10-game homestand with the third in a four-game series Wednesday night at against the Los Angeles Dodgers.