Lo, 27, owns a 3.24 ERA (6ER/16.2IP) with six saves in 17 appearances combined between Corpus Christi (8g), Class A Quad Cities (3g) and Class A Tri-City (6g) this season. He has 20 strikeouts in 16.2 innings, while allowing a .234 opponent's batting average. He joined Corpus Christi on July 10 and has tossed 7.2 scoreless innings over his last seven appearances. Lo was one of the final cuts this Spring Training, as he posted a 2.08 ERA (2ER/8.2IP) in nine appearances for the Astros.
Zeid, 26, has gone 4-1 with 13 saves and a 3.50 ERA (17ER/43.2IP) in 43 relief appearances for Oklahoma City this season. He has served as the RedHawks closer since June 15 and is 13-for-15 in save opportunities in 2013. Zeid has posted 53 strikeouts in his 43.2 innings pitched, while allowing a .231 opponent's batting average. Like Lo, Zeid also pitched well in Spring Training, making six appearances and posting a 1.50 ERA (1ER/6IP) for the Astros.
Ambriz, 29, has made 43 relief appearances for the Astros this season over two stints with the club. He most recently returned to Houston on July 19.
Lo and Zeid are the ninth and 10th players to make their first appearance on the Astros Major League roster this season. After the Zeid move, the Astros now have 37 players on their 40-man roster.